clk: clps711x: Add DT bindings documentation

* Clock bindings for the Cirrus Logic CLPS711X CPUs

Required properties:

- compatible       : Shall contain "cirrus,clps711x-clk".

- reg              : Address of the internal register set.

- startup-frequency: Factory set CPU startup frequency in HZ.

- #clock-cells     : Should be <1>.

The clock consumer should specify the desired clock by having the clock

ID in its "clocks" phandle cell. See include/dt-bindings/clock/clps711x-clock.h

for the full list of CLPS711X clock IDs.

Example:

	clks: clks@80000000 {

		#clock-cells = <1>;

		compatible = "cirrus,ep7312-clk", "cirrus,clps711x-clk";

		reg = <0x80000000 0xc000>;

		startup-frequency = <73728000>;

	};