mediatek: mt7622: add support for ELECOM WRC-X3200GST3

ELECOM WRC-X3200GST3 is a 2.4/5 GHz band 11ax (Wi-Fi 6) router, based on
MT7622B.

Specifications:

- SoC		: MediaTek MT7622B
- RAM		: DDR3 512 MiB (Nanya NT5CC256M16ER-EK)
- Flash		: SPI-NAND 128 MiB (Winbond W25N01GVZEIG)
- WLAN		: 2.4/5 GHz 4T4R
  - 2.4 GHz	: MediaTek MT7622B (SoC)
  - 5 GHz	: MediaTek MT7915A
- Ethernet	: 5x 10/100/1000 Mbps
  - Switch	: MediaTek MT7531
- LEDs/Keys	: 6x/4x (2x buttons, 1x slide-switch)
- UART		: through-hole on PCB
  - J19: 3.3V, GND, TX, RX from power jack side
  - 115200n8
- Power		: 12 VDC, 1.5 A

Flash instruction using factory image:

1. Boot WRC-X3200GST3 normally with "Router" mode
2. Access to "http://192.168.2.1/" and open firmware update page
   ("ファームウェア更新")
3. Select the OpenWrt factory image and click apply ("適用") button
4. Wait ~120 seconds to complete flashing

MAC Addresses:

LAN    : 04:AB:18:xx:xx:77 (Factory, 0x7FFF4 (hex))
WAN    : 04:AB:18:xx:xx:78 (Factory, 0x7FFFA (hex))
2.4 GHz: 04:AB:18:xx:xx:79 (Factory, 0x4     (hex))
5 GHz  : 04:AB:18:xx:xx:7A (none)

Note:

- currently, there is no "phy1tpt" trigger for 5 GHz wlan (MT7915) in
  "trigger" file of LEDs, use "phy1radio" trigger instead

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