ramips: mt7621: Add DNA Valokuitu Plus EX400

ramips: mt7621: Add DNA Valokuitu Plus EX400

Specifications:
- Device: DNA Valokuitu Plus EX400
- SoC: MT7621A
- Flash: 256MB NAND
- RAM: 256MB
- Ethernet: Built-in, 2 x 1GbE
- Wifi: MT7603 2.4 GHz, MT7615 5 GHz (4x internal antennas)
- USB: 1x 3.0
- LED: 1x green/red, 1x green
- Buttons: Reset

MAC addresses:
- LAN:     u-boot 'ethaddr' (label)
- WAN:     label + 1
- 2.4 GHz: label + 6
- 5 GHz:   label + 7

Serial:
 There is a black block connector next to the red ethernet connector. It
 is accessible also through holes in the casing.

Pinout (TTL 3.3V)
 +---+---+
 |Tx |Rx |
 +---+---+
 |Vcc|Gnd|
 +---+---+

Firmware:
 The vendor firmware is a fork of OpenWrt (Reboot) with a kernel version
 4.4.93. The flash is arranged as below and there is a dual boot
 mechanism alternating between rootfs_0 and rootfs_1.

 +-------+------+------+-----------+-----------+
 |       | env1 | env2 | rootfs_0  |  rootfs_1 |
 |       +------+------+-----------+-----------+
 |       |         UBI volumes                 |
 +-------+-------------------------------------+
 |U-Boot |             UBI                     |
 +-------+-------------------------------------+
 |mtd0   |             mtd1                    |
 +-------+-------------------------------------+
 |                     NAND                    |
 +---------------------------------------------+

 In OpenWrt rootfs_0 will be used as a boot partition that will contain the
 kernel and the dtb. The squashfs rootfs and overlay are standard OpenWrt
 behaviour.

 +-------+------+------+-----------+--------+------------+
 |       | env1 | env2 | rootfs_0  | rootfs | rootfs_data|
 |       +------+------+-----------+--------+------------+
 |       |         UBI volumes                           |
 +-------+-----------------------------------------------+
 |U-Boot |             UBI                               |
 +-------+-----------------------------------------------+
 |mtd0   |             mtd1                              |
 +-------+-----------------------------------------------+
 |                     NAND                              |
 +-------------------------------------------------------+

U-boot:
 With proper serial access booting can be halted to U-boot by pressing any
 key. TFTP and flash writes are available, but only the first one has been
 tested.

 NOTE: Recovery mode can be accessed by holding down the reset button while
 powering on the device. The led 'Update' will show a solid green light
 once ready. A web server will be running at 192.168.1.1:80 and it will
 allow flashing a firmware package. You can cycle between rootfs_0 and
 rootfs_1 by pressing the reset button once.

Root password:
 With the vendor web UI create a backup of your settings and download the
 archive to your computer. Within the archive in the file
 /etc/shadow replace the password hash for root with that of a password you
 know. Restore the configuration with the vendor web UI and you will have
 changed the root password.

SSH access:
 You might need to enable the SSH service for LAN interface as by default
 it's enabled for WAN only.

Installing OpenWrt:
 With the vendor web UI install the OpenWrt factory image. Alternatively,
 ssh to the device and use sysupgrade -n from cli.

 Finalize by installing the OpenWrt sysupgrade image to get a fully
 functioning system.

Reverting to the vendor firmware:

 Boot with OpenWrt initramfs image
  - Remove volumes rootfs_0, rootfs and rootfs_data and create vendor
    volumes.

    ubirmvol /dev/ubi0 -n 2
    ubirmvol /dev/ubi0 -n 3
    ubirmvol /dev/ubi0 -n 4
    ubimkvol /dev/ubi0 -N rootfs_0 -S 990
    ubimkvol /dev/ubi0 -N rootfs_1 -S 990

 Power off and enter to the U-boot recovery to install the vendor
 firmware.

Known issues:
 - MACs for wifi are stored in currently unknown place but it seems
   to persist over power-off. They might be stored on the chip.

Signed-off-by: Mauri Sandberg <maukka@ext.kapsi.fi>
[rmilecki: try NVMEM for MACs]
Signed-off-by: Rafał Miłecki <rafal@milecki.pl>