ar71xx: add support for GL.iNet GL-AR750

ar71xx: add support for GL.iNet GL-AR750

GL.iNet GL-AR750 is a small size, dual-band (AC750) router, based on
Qualcomm/Atheros QCA9531 v2 + QCA9887. FCC ID: 2AFIW-AR750.

Specification:

- 650/597/216 MHz (CPU/DDR/AHB)
- 128 MB of RAM (DDR2)
- 16 MB of FLASH (SPI NOR)
- 3x 10/100 Mbps Ethernet
- 2T2R 2.4 GHz (QCA9531)
- 1T1R 5 GHz (QCA9887)
- 1x USB 2.0 (power controlled by GPIO)
- 1x microSD (GL857L)
- 3x LED (all driven by GPIO)
- 1x button (reset)
- 1x 2-pos switch
- header for optional PoE module
- 1x micro USB for main power input
- UART + I2C header on PCB

Flash instruction:

Vendor firmware is based on OpenWrt/LEDE. GUI or sysupgrade can be used
to flash OpenWrt/LEDE firmware.

Signed-off-by: Piotr Dymacz <pepe2k@gmail.com>