dt-bindings: clock: add DT binding for the Xtal clock on Armada 3700

This commit adds the DT binding documentation for the the Xtal clock on
Armada 3700 used in the Marvell Armada 3700 SoCs.
Signed-off-by: NGregory CLEMENT <gregory.clement@free-electrons.com>
Acked-by: NRob Herring <robh@kernel.org>
Signed-off-by: NStephen Boyd <sboyd@codeaurora.org>

Documentation/devicetree/bindings/clock/armada3700-xtal-clock.txt

+* Xtal Clock bindings for Marvell Armada 37xx SoCs
+
+Marvell Armada 37xx SoCs allow to determine the xtal clock frequencies by
+reading the gpio latch register.
+
+This node must be a subnode of the node exposing the register address
+of the GPIO block where the gpio latch is located.
+
+Required properties:
+- compatible : shall be one of the following:
+	"marvell,armada-3700-xtal-clock"
+- #clock-cells : from common clock binding; shall be set to 0
+
+Optional properties:
+- clock-output-names : from common clock binding; allows overwrite default clock
+	output names ("xtal")
+
+Example:
+gpio1: gpio@13800 {
+	compatible = "marvell,armada-3700-gpio", "syscon", "simple-mfd";
+	reg = <0x13800 0x1000>;
+
+	xtalclk: xtal-clk {
+		compatible = "marvell,armada-3700-xtal-clock";
+		clock-output-names = "xtal";
+		#clock-cells = <0>;
+	};
+};