mediatek: add support for Buffalo WSR-2533DHP3

mediatek: add support for Buffalo WSR-2533DHP3

Buffalo WSR-2533DHP3 is a 2.4/5 GHz band 11ac router, based on
MT7622B.

Specification:

- SoC         : MediaTek MT7622B
- RAM         : DDR3 256 MiB
- Flash       : SPI-NAND 128 MiB (Winbond W25N01GVZEIG)
- WLAN        : 2.4/5 GHz 4T4R
  - 2.4 GHz   : MediaTek MT7622B (SoC)
  - 5 GHz     : MediaTek MT7615
- 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-2533DHP3 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.

Tested:

- initramfs boot
- factory.bin installation from OEM WebUI
- LAN port assignment
- WAN DHCP
- NAT routing
- 2.4 GHz Wi-Fi
- 5 GHz Wi-Fi
- LEDs
- Buttons

MAC Addresses:

LAN    : 58:27:8C:xx:xx:90 (board_data, mac (text))
WAN    : 58:27:8C:xx:xx:90 (board_data, mac (text))
2.4 GHz: 58:27:8C:xx:xx:91
5 GHz  : 58:27:8C:xx:xx:94

Signed-off-by: Taiga Ogawa <zectaiga@gmail.com>
Link: https://github.com/openwrt/openwrt/pull/23497
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>