ath79: add support for devolo WiFi pro 1750c

ath79: add support for devolo WiFi pro 1750c

Hardware
--------
CPU:   Qualcomm Atheros QCA9558
RAM:   128M DDR2
FLASH: 16MiB
ETH:   1x Atheros AR8035 (PoE in)
WiFi2: QCA9558 3T3R
WiFi5: QCA9880 3T3R
BTN:   1x Reset
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 square pad)
       Header is located next to reset-button

There is also a Micro-B USB-port present but this only seems to be a
dummy as the circuit next to it is not present (at least in my unit).

It is also not mentioned in the devolo manual.

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 the one set in the web interface.

2. Install OpenWRT with

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

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