ar71xx: add support for GainStrong Oolite V5.2

ar71xx: add support for GainStrong Oolite V5.2

GainStrong Oolite V5.2 is a small (39.5 x 78 mm), dual-band system on
module, based on Qualcomm QCA9531 + QCA9887. All QCA9531 SOC GPIOs are
available on castellated pins.

Specification:

- 650/400/200 MHz (CPU/DDR/AHB)
- 64/128 MB of RAM (DDR2)
- 16 MB of FLASH (SPI NOR)
- 5x 10/100 Mbps Ethernet (all ports available on castellated pins)
- 2T2R 2.4 GHz (QCA9531), with ext. PA and LNA
- 1T1R 5 GHz (QCA9887) with ext. FEM (SKY85710-11)
- 3x U.FL
- 1x USB 2.0

The dedicated development board includes 5 FE ports, USB 2.0 port, two
buttons (one directly connected to SOC reset input), header with all
GPIOs and several LEDs.

Flash instruction:

Vendor firmware is based on OpenWrt. LuCI or sysupgrade can be used to
flash OpenWrt firmware.

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