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/dts/qca9558_devolo_dvl1750c.dts

@@ -0,0 +1,46 @@
+// SPDX-License-Identifier: GPL-2.0-or-later OR MIT
+/dts-v1/;
+
+#include <dt-bindings/gpio/gpio.h>
+#include <dt-bindings/input/input.h>
+
+#include "qca9558_devolo_dvl1xxx.dtsi"
+
+/ {
+	compatible = "devolo,dvl1750c", "qca,qca9557";
+	model = "devolo WiFi pro 1750c";
+
+	aliases {
+		led-boot = &status_blue;
+		led-failsafe = &status_red;
+		led-running = &status_blue;
+		led-upgrade = &status_red;
+	};
+
+	leds {
+		compatible = "gpio-leds";
+
+		status_blue: status_blue {
+			label = "dvl1750c:blue:status";
+			gpios = <&gpio 14 GPIO_ACTIVE_LOW>;
+			default-state = "on";
+		};
+
+		status_red: status_red {
+			label = "dvl1750c:red:status";
+			gpios = <&gpio 15 GPIO_ACTIVE_LOW>;
+		};
+	};
+
+	beeper {
+		compatible = "gpio-beeper";
+		gpios = <&gpio 21 GPIO_ACTIVE_HIGH>;
+	};
+};
+
+&gmac_config {
+	rxdv-delay = <3>;
+	rxd-delay = <3>;
+	txen-delay = <3>;
+	txd-delay = <3>;
+};