ath79: add support for Devolo WiFi Pro 1750e

ath79: add support for Devolo WiFi Pro 1750e

Hardware
--------
CPU:   Qualcomm Atheros QCA9558
RAM:   128M DDR2
FLASH: 16MiB
ETH:   1x Atheros AR8035 (PoE in)
       1x Atheros AR8033
WiFi2: QCA9558 3T3R (SiGE SE2565T 2.4 GHz power amp x3)
WiFi5: QCA9880 3T3R (Skyworks 5003L1 5 GHz power amp x3)
BTN:   1x Reset
       1x WPS
       1x USB eject
LED:   1x LED blue
       1x LED red
BEEP:  1x GPIO attached piezo beeper
UART:  3.3V GND TX RX (115200-N-8) (3.3V is pin closest to rear ports)
       Dupont 4 pin header
       Rear RJ45 serial port non-functional
USB:   1x v2.0

Installation
------------
Make sure you set a password for the root user as prompted on first
setup!

1. Upload OpenWRT sysupgrade image via SSH to the device.
Use /tmp as the destination folder on the device.
User is root, password previously set in the web interface.

2. Install OpenWRT with

> sysupgrade -n -F /tmp/<openwrt-image-name>

Signed-off-by: Django Armstrong <iamdjango@hotmail.com>