ralink: add support for netis WF-2881

netis WF-2881 is an MT7621AT based router with MT7602EN, MT7612EN.
It has 128MB DDR3, 128MB SLC NAND FLASH, 5-port Gbps switch and 1x USB 3.0.
The following patch adds support for this device.

this device only works on top of UBI.

Tested and working:
* ethernet
* both WiFi radios
* USB 3.0
* buttons (reset button)
* ethernet switch and USB diag LEDs
* UART
* GPIOs
* sysupgrade

Tested and not working
* failsafe

Signed-off-by: YounJae Rho <luxflow@live.com>

SVN-Revision: 47619