ramips: add support for TP-Link Archer C2

ramips: add support for TP-Link Archer C2

Specification:

- System-On-Chip: MT7620A
- CPU/Speed: 580 MHz
- Flash-Chip: Winbond 25Q64BVSIG
- Flash size: 8192 KiB
- RAM: 64 MiB
- Wireless No1: SoC-integrated: MT7620A 2.4GHz 802.11bgn
- Wireless No2: On-board chip: MT7610EN 5GHz 802.11ac
- Switch: RTL8367RB Gigabit Switch
- USB: Yes 1 x 2.0

Preparing a TFTP recovery image for initial flashing:

Currently the only method to install openwrt for the first time is via
TFTP download in u-boot. After first install you can use regular updates.
WARNING: This method also overwrites the bootloader partition!

Create a TFTP recovery image:

1) Download a stock TP-Link Firmware file here:
     https://www.tp-link.com/en/download/Archer-C2_V1.html#Firmware
2) Extract u-boot from the binary file:
     #> dd if=c2v1_stock_firmware.bin of=c2v1_uboot.bin bs=1 skip=512 count=131072
3) Now merge the sysupgrade image and the u-boot into one binary:
     #> cat c2v1_uboot.bin openwrt-squashfs-sysupgrade.bin > ArcherC2V1_tp_recovery.bin

The resulting image can be flashed via TFTP recovery mode.

Flash instructions:

1) To flash the recovery image, start a TFTP server from IP address
   192.168.0.66 and serve the recovery image named
   ArcherC2V1_tp_recovery.bin.
2) Connect your device to the LAN port, then press the WPS/Reset button
   and power it up. Keep pressing the WPS/Reset button for 10 seconds.
   It will try to download the recovery image and flash it.

It can take up to 20-25 minutes to finish. When it reaches 100%, the
router will reboot itself.

Signed-off-by: Serge Vasilugin <vasilugin@yandex.ru>
Signed-off-by: Franz Flasch <franz.flasch@gmx.at>