ramips: add support for Netgear R6220

ramips: add support for Netgear R6220

This patch adds support for the Netgear R6220, aka Netgear AC1200 and
R6220-100NAS.

Specification:
- SoC: MediaTek MT7621ST (880 MHz)
- Falsh: 128 MiB (Macronix MX30LF1G08AA-TI)
- RAM: 128 MiB (Nanya NT5CB64M16FP-DH)
- Wireless: MediaTek MT7603EN b/g/n , MediaTek MT7612EN an+ac
- LAN speed: 10/100/1000
- LAN ports: 4
- WAN speed: 10/100/1000
- WAN ports: 1
- Serial baud rate of Bootloader and factory firmware: 57600

Installation through telnet:
- Copy kernel.bin and rootfs.bin to a USB flash disk, plug to usb port
  on the router.
link: http://192.168.1.1/setup.cgi?todo=debug
  (login if required, default: admin password)
- You will see "Debug Enabled!"
- Telnet 192.168.1.1 and login with "root"
- ls /mnt/shares/ to find out path of your USB disk. 'myUdisk' for
  example.
- cd /mnt/shares/myUdisk
- mtd_write write rootfs.bin Rootfs
- mtd_write write kernel.bin Kernel
- reboot

nmrpflash can be used to recover to the netgear firmware if a broken
image was flashed.

Signed-off-by: Hanqing Wong <hquu@outlook.com>