ath79: add support for gl-ar750

ath79: add support for gl-ar750

This patch supports gl-ar750, which was previously supported by ar71xx.

Specification:
- SOC: QCA9531 (650MHz)
- Flash: 16 MiB (W25Q128FVSG)
- RAM: 128 MiB DDR2
- Ethernet: 10/100: 2xLAN + 10/100: 1xWAN
- Wireless: 2.4GHz (bgn) and 5GHz (ac)
- USB: 1x USB 2.0 port
- Switch: 1x switch
- Button: 1x reset button
- LED: 3x LEDS (white)

Flash instruction:
Support for sysupgrade directive upgrades, as well as luci upgrades.

Signed-off-by: Luochongjun <luochongjun@gl-inet.com>