ath79: add support for GL.iNet GL-X300B

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:

https://github.com/gl-inet/openwrt/commit/202e83a32ae308fbb70502b6dbe3bb0bf8b1fba9

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:

https://github.com/gl-inet/openwrt/commit/16c5708b207eb76ff19a040dc973e560d3d8074b

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