ramips: add support for ELECOM WRC-2533GHBK-I

ramips: add support for ELECOM WRC-2533GHBK-I

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

Specification:

- SoC		: MediaTek MT7621A
- RAM		: DDR3 128 MiB
- Flash		: SPI-NOR 16 MiB
- WLAN		: 2.4/5 GHz 4T4R (2x MediaTek MT7615)
- Ethernet	: 10/100/1000 Mbps x5
  - Switch	: MediaTek MT7530 (SoC)
- LED/keys	: 4x/3x (2x buttons, 1x slide-switch)
- UART		: through-hole on PCB
  - J4: 3.3V, RX, GND, TX from SoC side
  - 57600n8
- Power		: 12VDC, 1.5A

Flash instruction using factory image:

1. Boot WRC-2533GHBK-I normally
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 ~150 seconds to complete flashing

MAC addresses:

LAN	: BC:5C:4C:xx:xx:89 (Config, ethaddr (text))
WAN	: BC:5C:4C:xx:xx:88 (Config, wanaddr (text))
2.4GHz	: BC:5C:4C:xx:xx:8A (Factory, 0x4    (hex))
5GHz	: BC:5C:4C:xx:xx:8B (Factory, 0x8004 (hex))

Signed-off-by: INAGAKI Hiroshi <musashino.open@gmail.com>
Reviewed-by: Sungbo Eo <mans0n@gorani.run>