ath79: add support for GL.iNet GL-X300B

The GL-X300B is a industrial 4G LTE router based on the Qualcomm
QCA9531 SoC.

Specifications:
 - Qualcomm QCA9531 @ 650 MHz
 - 128 MB of RAM
 - 16 MB of SPI NOR FLASH
 - 2x 10/100 Mbps Ethernet
 - 2.4GHz 802.11b/g/n
 - 1x USB 2.0 (vbus driven by GPIO)
 - 4x LED, driven by GPIO
 - 1x button (reset)
 - 1x mini pci-e slot (vcc driven by GPIO)
 - RS-485 Serial Port (untested)

Flash instructions:

This firmware can be flashed using either sysupgrade from the GL.iNet
firmware or the recovery console as follows:

 - Press and hold the reset button
 - Connect power to the router, wait five seconds
 - Manually configure 192.168.1.2/24 on your computer, connect to
   192.168.1.1
 - Upload the firmware image using the web interface

RS-485 serial port is untested and may depend on the following commit in
the GL.iNet repo:

202e83a32a

MAC addresses as verified by OEM firmware:

vendor   OpenWrt   address
WAN      eth0      label
LAN      eth1      label + 1
2g       phy0      label + 2

The label MAC address was found in the art partition at 0x0

Based on vendor commit:

16c5708b20

Signed-off-by: John Marrett <johnf@zioncluster.ca>

target/linux/ath79/generic/base-files/etc/board.d/01_leds

@@ -219,6 +219,7 @@ qxwlan,e750a-v4-16m)
 	ucidef_set_led_switch "lan" "LAN" "green:lan" "switch0" "0x02"
 	ucidef_set_led_netdev "wan" "WAN" "green:wan" "eth1"
 	;;
+glinet,gl-x300b|\
 glinet,gl-x750)
 	ucidef_set_led_netdev "wan" "WAN" "green:wan" "eth1"
 	;;

target/linux/ath79/generic/base-files/etc/board.d/02_network

@@ -272,6 +272,11 @@ ath79_setup_interfaces()
 		ucidef_add_switch "switch0" \
 			"0@eth0" "1:lan" "2:lan"
 		;;
+	glinet,gl-x300b)
+		ucidef_set_interface_wan "eth1"
+		ucidef_add_switch "switch0" \
+			"0@eth0" "4:lan"
+		;;
 	iodata,etg3-r|\
 	iodata,wn-ac1167dgr|\
 	iodata,wn-ac1600dgr|\