ramips: add support for TP-Link Archer C5 v4

ramips: add support for TP-Link Archer C5 v4

TP-Link Archer C5 v4 is a dual band router with 5 GbE ports
Advertised as AC1200 for its 867Mbps (2x2) 5GHz band
and 300 Mbps (2x2) 2.4GHz band.

Specs:
- SoC: MediaTek MT7620A
- Ethernet: 5x GbE ports (Realtek RTL8367S)
- Wireless 2.4GHz: MediaTek MT7620A
- Wireless 5GHz: MediaTek MT7612E
- RAM: 64MiB
- ROM: 8MiB (GD25Q64CSIG)
- 1 USB 2.0 port
- 2 Buttons (WPS and reset)
- 8 LEDs

Flash instructions:
Currently one has to install OpenWrt only via the serial console

1. Rename the factory.bin to to test.bin
2. start a TFTP server from IP address 192.168.0.225 and serve the image named test.bin
3. connect your device to the LAN port
4. power up the router and press 4 on the console to stop the boot process.
5. enter the following commands on the router console
	tftp 0x80060000 test.bin
	erase tplink 0x20000 0x7a0000
	cp.b 0x80060000 0x20000 0x7a0000
	reset

Signed-off-by: DENG Qingfang <dengqf6@mail2.sysu.edu.cn>