ramips: mt7621: Add Arcadyan WE420223-99 support

ramips: mt7621: Add Arcadyan WE420223-99 support

The Arcadyan WE420223-99 is a WiFi AC simultaneous dual-band access
point distributed as Experia WiFi by KPN in the Netherlands. It features
two ethernet ports and 2 internal antennas.

Specifications
--------------
SOC   : Mediatek MT7621AT
ETH   : Two 1 gigabit ports, built into the SOC
WIFI  : MT7615DN
BUTTON: Reset
BUTTON: WPS
LED   : Power (green+red)
LED   : WiFi (green+blue)
LED   : WPS (green+red)
LED   : Followme (green+red)
Power : 12 VDC, 1A barrel plug

Winbond variant:
RAM   : Winbond W631GG6MB12J, 1GBIT DDR3 SDRAM
Flash : Winbond W25Q256JVFQ, 256Mb SPI
U-Boot: 1.1.3 (Nov 23 2017 - 16:40:17), Ralink 5.0.0.1

Macronix variant:
RAM   : Nanya NT5CC64M16GP-DI, 1GBIT DDR3 SDRAM
Flash : MX25l25635FMI-10G, 256Mb SPI
U-Boot: 1.1.3 (Dec  4 2017 - 11:37:57), Ralink 5.0.0.1

Serial
------
The serial port needs a TTL/RS-232 3V3 level converter! The Serial
setting is 57600-8-N-1. The board has an unpopulated 2.54mm straight pin
header.

The pinout is: VCC (the square), RX, TX, GND.

Installation
------------
See the Wiki page [1] for more details, it comes down to:

1. Open the device, take off the heat sink
2. Connect the SPI flash chip to a flasher, e.g. a Raspberry Pi. Also
   connect the RESET pin for stability (thanks @FPSUsername for reporting)
3. Make a backup in case you want to revert to stock later
4. Flash the squashfs-factory.trx file to offset 0x50000 of the flash
5. Ensure the bootpartition variable is set to 0 in the U-Boot
   environment located at 0x30000

Note that the U-Boot is password protected, this can optionally be
removed. See the forum [2] for more details.

MAC Addresses(stock)
--------------------
+----------+------------------+-------------------+
| use      | address          | example           |
+----------+------------------+-------------------+
| Device   | label            | 00:00:00:11:00:00 |
| Ethernet | + 3              | 00:00:00:11:00:03 |
| 2g       | + 0x020000f00001 | 02:00:00:01:00:01 |
| 5g       | + 1              | 00:00:00:11:00:01 |
+----------+------------------+-------------------+

The label address is stored in ASCII in the board_data partition

Notes
-----
- This device has a dual-boot partition scheme, but OpenWRT will claim
  both partitions for more storage space.

Known issues
------------
- 2g MAC address does not match stock due to missing support for that in
  macaddr_add
- Only the power LED is configured by default

References
----------
[1] https://openwrt.org/inbox/toh/arcadyan/astoria/we420223-99
[2] https://forum.openwrt.org/t/adding-openwrt-support-for-arcadyan-we420223-99-kpn-experia-wifi/132653

Acked-by: Arınç ÜNAL <arinc.unal@arinc9.com>
Signed-off-by: Harm Berntsen <git@harmberntsen.nl>