ramips: add support for ELECOM WRC-1750GST2

ramips: add support for ELECOM WRC-1750GST2

ELECOM WRC-1750GST2 is a 2.4/5 GHz band 11ac (Wi-Fi 5) router, based on
MT7621A.

Specification:

- SoC		: MediaTek MT7621A
- RAM		: DDR3 256 MiB (NT5CC128M16JR-EK)
- Flash		: SPI-NOR 32 MiB (MX25L25645GMI-08G)
- WLAN		: 2.4/5 GHz 3T3R (2x MediaTek MT7615)
- Ethernet	: 10/100/1000 Mbps x5
  - Switch	: MediaTek MT7530 (SoC)
- LEDs/Keys	: 4x/6x (2x buttons, 1x slide-switch)
- UART		: through-hole on PCB
  - J4: 3.3V, GND, TX, RX, from ethernet port side
  - 57600n8
- Power		: 12 VDC, 1.5 A

Flash instruction using factory image:

1. Boot WRC-1750GST2 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:23 (Factory, 0xE000 (hex))
WAN	: 04:AB:18:xx:xx:24 (Factory, 0xE006 (hex))
2.4GHz	: 04:AB:18:xx:xx:25 (Factory, 0x4    (hex))
5GHz	: 04:AB:18:xx:xx:26 (Factory, 0x8004 (hex))

Signed-off-by: INAGAKI Hiroshi <musashino.open@gmail.com>
(cherry picked from commit b3ca1f30efd7bb96a22f72b766d302e552265276)