ramips: add support for STORYLiNK SAP-G3200U3

ramips: add support for STORYLiNK SAP-G3200U3

STORYLiNK SAP-G3200U3 is an AC1200 router based on MT7621AT+MT7602EN+MT7612EN.
It has 128MB DDR3, 8MB NOR FLASH, 5-port Gbps switch and 1x USB 3.0.

The following patch adds support for this device.

Tested and working:
* ethernet
* both WiFi radios
* USB 3.0
* buttons
* ethernet switch and USB diag LEDs
* UART
* GPIOs

Tested and not working:
* LEDs for WiFi radios (connected with WiFi chips, not supported in mt76?)
* failsafe mode (known problem, needs workaround like other MTK based devices)

More information in Wiki: http://wiki.openwrt.org/toh/storylink/sap-g3200u3

Signed-off-by: Piotr Dymacz <pepe2k@gmail.com>
git-svn-id: svn://svn.openwrt.org/openwrt/trunk@46454 3c298f89-4303-0410-b956-a3cf2f4a3e73