ath79: add support for Xiaomi Mi Router 4Q

ath79: add support for Xiaomi Mi Router 4Q

Hardware
--------
CPU:   Qualcomm Atheros QCA9561
RAM:   64M DDR2
FLASH: 16M SPI-NOR
ETH:   1x WAN - 2x LAN
WiFi:  QCA9561 3T3R
BTN:   1x Reset - 1x WPS
LED:   1x Blue - 1x Red - 1x Yellow
UART:  TX - GND - RX - VCC (From ethernet port)
       115200n8 - 3.3V

Installation
------------
1. Connect to the device via UART.

2. Interrupt the U-Boot on power-on by pressing enter when prompted.

3. Connect you computer to one of the routers LAN ports.
   Assign yourself the IP 192.168.31.10/24.
   Copy the OpenWRT initramfs image to a tftp server root directory.
   Rename the image to 'x4q.bin'.

4. Load the initramfs image to the router by executing following command
   in U-Boot. The image will boot afterwards.

   > tftpboot 0x81000000 x4q.bin; bootm

5. SCP the sysupgrade-image into '/tmp'.
   Remember to assign yourself an IP in 192.168.1.0/24 for this step!

6. Install OpenWRT permanently by executing

   > sysupgrade -n /tmp/<OpenWRT-sysupgrade-image>

Signed-off-by: David Bauer <mail@david-bauer.net>