ath79: add support for GL.iNet GL-XE300

ath79: add support for GL.iNet GL-XE300

The GL.iNet GL-XE300 is a 4G LTE Wireless router, based on QCA9531 SoC.

Specifications:

 - SoC: QCA9531 (650MHz)
 - RAM: DDR2 128M
 - Flash: SPI NOR 16M + SPI NAND 128M
 - WiFi: 2.4GHz with 2 antennas
 - Ethernet:
   - 1x LAN (10/100M)
   - 1x WAN (10/100M)
 - LTE:
 - USB: 1x USB 2.0 port
 - UART:
   - 3.3V, TX, RX, GND / 115200 8N1

MAC addresses as verified by OEM firmware:

 use    address   source
 LAN    *:c5      art 0x0 (label)
 WAN    *:c6      label + 1
 WLAN   *:c7      art 0x1002

Installation via U-Boot rescue:

1. Press and hold reset and power buttons simultaneously
2. Wait for the LAN led to blink 5 times
3. Release reset and power buttons
4. The rescue page is accessible via http://192.168.1.1
5. Select the OpenWrt factory image and start upgrade
6. Wait for the router to flash new firmware and reboot

Revert to stock firmware:

 i. Download the stock firmware from GL.Inet website
 ii. Use the same method explained above to flash the stock firmware

Signed-off-by: Victorien Molle <victorien.molle@wifirst.fr>
[update commit message]
Signed-off-by: David Bauer <mail@david-bauer.net>