ar71xx: add support for GL.iNet GL-AR750S

This patch adds supports for GL-AR750S.

Specification:
- SOC: QCA9563 (775MHz)
- Flash: 16 MiB (W25Q128FVSG)
- RAM: 128 MiB DDR2
- Ethernet: 2x 1Gbps LAN + 1x 1Gbps WAN
- Wireless: 2.4GHz (bgn) and 5GHz (ac)
- USB: 1x USB 2.0 port
- Button: 1x switch button, 1x reset button
- LED: 3x LEDS (green)

Flash instruction:
Apply factory image via web-gui.

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

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

@@ -727,6 +727,9 @@ ar71xx_board_detect() {
 	*"GL-AR750")
 		name="gl-ar750"
 		;;
+	*"GL-AR750S")
+		name="gl-ar750s"
+		;;
 	*"GL-CONNECT INET v1")
 		name="gl-inet"
 

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

@@ -260,6 +260,7 @@ platform_check_image() {
 	gl-ar300m|\
 	gl-ar300|\
 	gl-ar750|\
+	gl-ar750s|\
 	gl-domino|\
 	gl-mifi|\
 	gl-usb150|\