ar71xx:add support for E750A v4

ar71xx:add support for E750A v4

Qxwlan E750A v4 is based on Qualcomm QCA9344.

Specification:

 - 560/450/225 MHz (CPU/DDR/AHB)
 - 128 MB of RAM (DDR2)
 - 8/16 MB of FLASH (SPI NOR)
 - 2T2R 5G GHz (AR9344)
 - 2x 10/100 Mbps Ethernet (one port with PoE support)
 - 1x miniPCIe slot (USB 2.0 bus only)
 - 7x LED (6 driven by GPIO)
 - 1x button (reset)
 - 1x DC jack for main power input (9-48 V)
 - UART (J23) and LEDs (J2) headers on PCB

Flash instruction (using U-Boot CLI and tftp server):

 - Configure PC with static IP 192.168.1.10 and tftp server.
 - Rename "sysupgrade" filename to "firmware.bin" and place it in tftp
   server directory.
 - Connect PC with one of RJ45 ports, power up the board and press
   "enter" key to access U-Boot CLI.
 - Use the following command to update the device to OpenWrt: "run lfw".

Flash instruction (using U-Boot web-based recovery):

 - Configure PC with static IP 192.168.1.xxx(2-254)/24.
 - Connect PC with one of RJ45 ports, press the reset button, power up
   the board and keep button pressed for around 6-7 seconds, until LEDs
   start flashing.
 - Open your browser and enter 192.168.1.1, select "sysupgrade" image
   and click the upgrade button.

Signed-off-by: 张鹏 <sd20@qxwlan.com>