arm64: dts: marvell: Add xtal clock support for Armada 3700

The configuration of the clock depend of the gpio latch. This information
is stored in the gpio block registers. That's why the block is shared
using a syscon node.
Signed-off-by: NGregory CLEMENT <gregory.clement@free-electrons.com>

arch/arm64/boot/dts/marvell/armada-37xx.dtsi

@@ -105,6 +105,18 @@
 				status = "disabled";
 			};
 
+			gpio1: gpio@13800 {
+				compatible = "marvell,mvebu-gpio-3700",
+				"syscon", "simple-mfd";
+				reg = <0x13800 0x500>;
+
+				xtalclk: xtal-clk {
+					compatible = "marvell,armada-3700-xtal-clock";
+					clock-output-names = "xtal";
+					#clock-cells = <0>;
+				};
+			};
+
 			usb3: usb@58000 {
 				compatible = "marvell,armada3700-xhci",
 				"generic-xhci";