mediatek: add support Teralink TL3020

mediatek: add support Teralink TL3020

This commit adds support for Teralink TL3020 256mb wireless router.

Specification
-------------
- SoC       : MediaTek MT7981B dual-core ARM Cortex-A53
- RAM       : DDR3 256 MiB (ESMT M15T2G16128A, DDR3-1866)
- Flash     : SPI-NAND 256 MiB (Winbond W25M02GV)
- WLAN      : MediaTek MT7976 2.4/5 GHz Wi-Fi 6
- Ethernet  : MediaTek MT7531 switch, 1x WAN + 3x LAN 10/100/1000 Mbps
              (2.5GbE uplink via gmac0)
- USB       : 1x USB 2.0 Type-A (power enable on GPIO 14)
- Buttons   : WPS, Reset
- LEDs      : 7x blue, 1x red
- UART      : 4-pin header on PCB
  - assignment  : 3.3V, GND, TX, RX
  - settings    : 115200n8
- Power     : 12 VDC, 1.5 A (barrel jack)

MAC addresses
-------------
EEPROM MAC is same for all the devices, but there is a `config2` partition
with unique MAC addresses:

| Interface | MAC source                                 |
|-----------|--------------------------------------------|
| LAN       | config2, offset 0x707 (ASCII, "lanmac")    |
| WAN       | config2, offset 0x787 (ASCII, "wanmac")    |
| WLAN 2.4G | config2, offset 0x68a (ASCII, "wifi2gmac") |
| WLAN 5G   | config2, offset 0x60a (ASCII, "wifi5gmac") |

The LAN MAC (gmac0) is the one printed on the device label.

Installation
------------
Factory bootloader is locked to 64mb partition layout, so OpenWrt U-Boot
has to be flashed first. Steps 1, 2 and 4 are run from a shell on the
stock firmware, where the 'bl2' and 'fip' partitions are writable - in
OpenWrt they are read-only, so the procedure cannot be repeated from
OpenWrt. Back up both partitions before overwriting them:

    mtd read bl2 /tmp/bl2.bin
    mtd read fip /tmp/fip.bin

1. Upload OpenWrt 'bl31-uboot.fip' and 'preloader.bin' images to the /tmp
   dir of the router using scp protocol
2. Write fip and bl2 (replace bootloader):

    mtd write /tmp/openwrt-mediatek-filogic-teralink_tl3020-256mb-bl31-uboot.fip fip
    mtd write /tmp/openwrt-mediatek-filogic-teralink_tl3020-256mb-preloader.bin bl2

3. Place OpenWrt
   'openwrt-mediatek-filogic-teralink_tl3020-256mb-initramfs-recovery.itb'
   image on the tftp server (IP: 192.168.1.254)
4. Erase 'ubi' partition and reboot the router:

    mtd erase ubi
    reboot

5. U-Boot automatically boots the OpenWrt recovery image from the tftp
   server to the RAM
6. Upload OpenWrt
   'openwrt-mediatek-filogic-teralink_tl3020-256mb-squashfs-sysupgrade.itb'
   image to the /tmp dir of the router (IP: 192.168.1.1) using scp
   protocol
7. Connect to the router using ssh and run:

    ubidetach -p /dev/mtd5; ubiformat /dev/mtd5 -y; ubiattach -p /dev/mtd5
    ubimkvol /dev/ubi0 -n 0 -N ubootenv -s 128KiB
    ubimkvol /dev/ubi0 -n 1 -N ubootenv2 -s 128KiB
    sysupgrade -n openwrt-mediatek-filogic-teralink_tl3020-256mb-squashfs-sysupgrade.itb

Recovery
--------
1. Place OpenWrt
   'openwrt-mediatek-filogic-teralink_tl3020-256mb-initramfs-recovery.itb'
   image on the tftp server (IP: 192.168.1.254)
2. Press "Reset" button and power on the router. After ~10 sec release
   the button.
3. Use OpenWrt initramfs system for recovery

Reverting to stock firmware requires writing the 'bl2' and 'fip' backups
taken above back with 'mtd write', then flashing the stock image; without
those backups there is no way back, because the stock bootloader cannot
be downloaded from the vendor.

Signed-off-by: Fil Dunsky <filipp.dunsky@gmail.com>
Link: https://github.com/openwrt/openwrt/pull/24518
Signed-off-by: Jonas Jelonek <jelonek.jonas@gmail.com>