mediatek: add support for ELECOM WRC-X3000GS3

mediatek: add support for ELECOM WRC-X3000GS3

ELECOM WRC-X3000GS3 is a 2.4/5 GHz band 11ax (Wi-Fi 6) router, based on
MT7981B.

Specification:

- SoC             : MediaTek MT7981B
- RAM             : DDR3 512 MiB (Winbond W634GU6QB-09)
- Flash           : SPI-NOR 128 MiB (Winbond W25N01GVZEIG)
- WLAN            : 2.4/5 GHz 2T2R (MediaTek MT7981B (SoC))
- Ethernet        : 5x 10/100/1000 Mbps
  - wan (phy)     : MediaTek MT7981B (SoC)
  - lan (switch)  : MediaTek MT7531A
- LEDs/Keys (GPIO): 8x/4x
- UART            : through-hole on PCB (J500)
  - assignment    : 3.3V, TX, RX, NC, GND from tri-angle marking
  - settings      : 115200n8
- Power           : 12 VDC, 1 A (Max. 12.8 W)

Flash instruction using factory.bin image:

1. Boot WRC-X3000GS3 in router mode normally
2. Access to the WebUI ("http://192.168.2.1/") on the device and open
   the firmware update page ("ファームウェア更新")
3. Select the OpenWrt factory.bin image and click apply ("適用") button
4. Wait ~120 seconds to complete flashing

Switching to the stock firmware:

1. Load the elecom.sh script

   . /lib/upgrade/elecom.sh

2. Check the current index of firmware partition

   mstc_rw_bootnum

3. Set the bootnum to opposite value between 1 and 2

   mstc_rw_bootnum <value>

   example:

   - step2 returned "1": mstc_rw_bootnum 2

   - step2 returned "2": mstc_rw_bootnum 1

4. Reboot

Notes:

- ELECOM sells (or sold) multiple models as AX3000 class with different
  hardwares:

  - WRC-X3000GS(N) : Lantiq(Intel) GRX350/GRX550
  - WRC-X3000GS(T)2: Qualcomm IPQ5018
  - WRC-X3000GS3   : MediaTek MT7981B

MAC Addresses:

LAN   : 38:97:A4:xx:xx:40 (Factory, 0x2A(hex)/Ubootenv,"ethaddr"(text))
WAN   : 38:97:A4:xx:xx:43 (Factory, 0x24(hex))
2.4GHz: 38:97:A4:xx:xx:41 (Factory, 0x4 (hex))
5GHz  : 38:97:A4:xx:xx:42 (Factory, 0xA (hex))

Signed-off-by: INAGAKI Hiroshi <musashino.open@gmail.com>
Link: https://github.com/openwrt/openwrt/pull/18976
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>