mediatek: add support for Buffalo WSR-3200AX4S

mediatek: add support for Buffalo WSR-3200AX4S

Buffalo WSR-3200AX4S is a 2.4/5 GHz band 11ax (Wi-Fi 6) router, based on
MT7622B.

Specification:

- SoC         : MediaTek MT7622B
- RAM         : DDR3 512 MiB
- Flash       : SPI-NAND 128 MiB (Winbond W25N01GVZEIG)
- WLAN        : 2.4/5 GHz 4T4R
  - 2.4 GHz   : MediaTek MT7622B (SoC)
  - 5 GHz     : MediaTek MT7915
- Ethernet    : 5x 10/100/1000 Mbps
  - Switch    : MediaTek MT7531
- LEDs/Keys   : 6x/5x (2x: buttons, 3x: slide-switches)
- UART        : through-hole on PCB (J4)
  - assignment: 3.3V, GND, TX, RX from tri-angle marking
  - settings  : 115200n8
- Power       : 12 VDC, 1.5 A

Flash instruction using factory.bin image:

1. Boot WSR-3200AX4S with "Router" mode
2. Access to "http://192.168.11.1/" and open firmware update page
   ("ファームウェア更新")
3. Select the OpenWrt factory.bin image and click update ("更新実行")
   button
4. Wait ~120 seconds to complete flashing

Note:

- This device has 2x OS images on flash. The first one will always be
  used for booting and the secondary is for backup.

- This support generates multiple factory*.bin image:

  - factory.bin      : for flashing from OEM WebUI
  - factory-uboot.bin: for flashing from U-Boot or clean installation
                       via sysupgrade (don't use for normal sysupgrade)

Known issues:

- Wi-Fi MAC addresses won't be applied to each adapter.

MAC Addresses:

LAN    : C4:3C:EA:xx:xx:60 (board_data, mac (text))
WAN    : C4:3C:EA:xx:xx:60 (board_data, mac (text))
2.4 GHz: C4:3C:EA:xx:xx:61
5 GHz  : C4:3C:EA:xx:xx:68

Signed-off-by: INAGAKI Hiroshi <musashino.open@gmail.com>