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>

target/linux/ar71xx/base-files/lib/ar71xx.sh

@@ -673,6 +673,9 @@ ar71xx_board_detect() {
 	*"GL-AR300M")
 		name="gl-ar300m"
 		;;
+	*"GL-AR750")
+		name="gl-ar750"
+		;;
 	*"GL-CONNECT INET v1")
 		name="gl-inet"
 

target/linux/ar71xx/base-files/lib/preinit/05_set_preinit_iface_ar71xx

@@ -17,6 +17,7 @@ set_preinit_iface() {
 	archer-c7|\
 	bhr-4grv2|\
 	dir-505-a1|\
+	gl-ar750|\
 	gl-inet|\
 	jwap003|\
 	pb42|\

target/linux/ar71xx/base-files/lib/upgrade/platform.sh

@@ -242,6 +242,7 @@ platform_check_image() {
 	gl-ar150|\
 	gl-ar300m|\
 	gl-ar300|\
+	gl-ar750|\
 	gl-domino|\
 	gl-mifi|\
 	gl-usb150|\