ramips: add support for ELECOM WMC-S1267GS2

ramips: add support for ELECOM WMC-S1267GS2

ELECOM WMC-S1267GS2 is a 2.4/5 GHz band 11ac (Wi-Fi 5) mesh extender,
based on MT7621A.

This device has almost the same hardware as WMC-M1267GST2.

Specification:

- SoC          : MediaTek MT7621A
- RAM          : DDR3 256 MiB (Nanya NT5CC128M16JR-EK)
- Flash        : SPI-NOR 32 MiB (Winbond W25Q256JVFIQ)
- WLAN         : 2.4/5 GHz 2T2R (MediaTek MT7615D)
- Ethernet     : 10/100/1000 Mbps x4
  - switch     : MediaTek MT7530 (SoC)
- LEDs/Keys    : 6x/6x
- UART         : through-hole on PCB ("J4")
  - arrangement: 3.3V, GND, TX, RX from tri-angle marking
  - settings   : 57600n8
- Power        : 12 VDC, 1 A

Flash instruction using factory image:

1. Boot WMC-S1267GS2 normally
2. Set IP address of the computer to 192.168.2.x
3. Access to "http://192.168.2.1/" and open firmware update page
   ("ファームウェア更新")
4. Select the OpenWrt factory image and click apply ("適用") button
5. Wait ~120 seconds to complete flashing

MAC addresses:

LAN    : 04:AB:18:xx:xx:9E (Factory, 0xFFF4 (hex))
2.4 GHz: 04:AB:18:xx:xx:9F
5 GHz  : 04:AB:18:xx:xx:A0 (Factory, 0x4 (hex))

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