ath79: add support for GL.iNet GL-S200

ath79: add support for GL.iNet GL-S200

Specifications:
SoC: QCA9531(650MHz)
RAM: DDR2 128M
Flash: SPI NOR 16M + SPI NAND 128M
WiFi: 2.4GHz with 2 antennas(WiFi/Thread)
Ethernet:
    1xLAN(10/100M)
    2xWAN(10/100M)
Button: 1x Reset Button
Switch: 1x Mode switch
LED: 1x Blue LED + 1x White LED + 1x Orange LED
IOT: Thread + ZigBee/Zwave

By uboot web failsafe:
Push the reset button for 5 seconds util the power led flash faster,
then use broswer to access http://192.168.1.1

Afterwards upgrade can use sysupgrade image.

Signed-off-by: Weiping Yang <weiping.yang@gl-inet.com>