ramips: add support for TP-Link Archer C50 v3

ramips: add support for TP-Link Archer C50 v3

TP-Link Archer C50 v3 is a router with 5-port FE switch and
non-detachable antennas. It's based on MediaTek MT7628N+MT7612E.

Specification:
- MediaTek MT7628N/N (580 Mhz)
- 64 MB of RAM
- 8 MB of FLASH
- 2T2R 2.4 GHz and 2T2R 5 GHz
- 5x 10/100 Mbps Ethernet
- 4x external, non-detachable antennas
- UART (J1) header on PCB (115200 8n1)
- 7x LED (GPIO-controlled*), 2x button, power switch

* WAN LED in this devices is a dual-color, dual-leads type which isn't
  (fully) supported by gpio-leds driver. This type of LED requires both
  GPIOs state change at the same time to select color or turn it off.
  For now, we support/use only the green part of the LED.

Flash instruction:

The only way to flash LEDE image in ArcherC50v3 is to use
tftp recovery mode in U-Boot:

1. Configure PC with static IP 192.168.0.66/24 and tftp server.
2. Rename "openwrt-ramips-mt7628-ArcherC50v3-squashfs-tftp-recovery.bin"
   to "tp_recovery.bin" and place it in tftp server directory.
3. Connect PC with one of LAN ports, press the reset button, power up
   the router and keep button pressed for around 6-7 seconds, until
   device starts downloading the file.
4. Router will download file from server, write it to flash and reboot.

Signed-off-by: Henryk Heisig <hyniu@o2.pl>