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

* 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>;

	};

};