ramips: add support for WINSTARS WS-WN536P3

ramips: add support for WINSTARS WS-WN536P3

WS-WN536P3 is a 2.4/5 GHz band 11ac router, based on MT7621DA.

Specification:

- SoC               : MediaTek MT7621DA
- RAM               : DDR3 128 MB
- Flash             : SPI-NOR 16MB
- WLAN              : 2.4/5 GHz 2T2R (MediaTek MT7603e/MT7613)
- Ethernet          : 5x 10/100/1000 Mbps
- LEDs              : 1x Sys 1x WIFI 5x Ethernet LEDs(connect to switch)
- Keys              : 1x Reset 1x WPS 1x Turbo
- USB               : x1
- UART              : 1xUART
- settings        : 115200n8

How to upgrade image in uboot

1. press reset button when boot for 5sec.
2. access 192.168.10.1 in web ui.
3. select image and upload

Signed-off-by: Sven Huang <1036456939@qq.com>