ramips: add support for ELECOM WRC-2533GS2

ramips: add support for ELECOM WRC-2533GS2

ELECOM WRC-2533GS2 is a 2.4/5 GHz band 11ac (Wi-Fi 5) router, based on
MT7621A.

Specification:

- SoC		: MediaTek MT7621A
- RAM		: DDR3 128 MiB (NT5CC64M16GP-DI)
- Flash		: SPI-NOR 16 MiB (MX25L12835FM2I-10G)
- WLAN		: 2.4/5GHz 4T4R (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-2533GS2 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:FB (Factory, 0xFFF4 (hex))
WAN	: 04:AB:18:xx:xx:FC (Factory, 0xFFFA (hex))
2.4 GHz	: 04:AB:18:xx:xx:FD (Factory, 0x4    (hex))
5 GHz	: 04:AB:18:xx:xx:FE (Factory, 0x8004 (hex))

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