ramips: add support for HiWiFi HC5962

This patch adds supports for the HiWiFi HC5962(gee4) http://www.hiwifi.com



Short specification:

 - MT7621AT + MT7612EN + 7603EN
 - 256MB DDR3 RAM
 - 128MB NAND flash
 - 1+3 x 1000M Ethernet
 - 1x USB 2.0 port. 1x USB 3.0 port.
 - reset button
 - UART pad on PCB (JP3: TX, RX, GND, 3.3V)

Flash instruction:

1, Download lede-ramips-mt7621-hc5962-squashfs-factory.bin
2, Login as root via SSH on 192.168.199.1 and then copy factory.bin(using wget or nc or...) to /tmp/
3, use the following commands:
   $ mtd write /tmp/lede-ramips-mt7621-hc5962-squashfs-factory.bin firmware
   $ mtd erase firmware_backup && reboot
After reboot you should be able to login as root via SSH on 192.168.1.1

Signed-off-by: ZengFei Zhang <zhangzengfei@kunteng.org>