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>

target/linux/ath79/image/generic.mk

@@ -146,6 +146,14 @@ define Device/devolo_dvl1200e
 endef
 TARGET_DEVICES += devolo_dvl1200e
 
+define Device/devolo_dvl1750c
+  ATH_SOC := qca9558
+  DEVICE_TITLE := devolo WiFi pro 1750c
+  DEVICE_PACKAGES := kmod-ath10k-ct ath10k-firmware-qca988x-ct
+  IMAGE_SIZE := 15936k
+endef
+TARGET_DEVICES += devolo_dvl1750c
+
 define Device/dlink_dir-825-b1
   ATH_SOC := ar7161
   DEVICE_TITLE := D-LINK DIR-825 B1