clk: dts: document pxa clock binding

* Clock bindings for Marvell PXA chips

Required properties:

- compatible: Should be "marvell,pxa-clocks"

- #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/.../pxa-clock.h).

Examples:

pxa2xx_clks: pxa2xx_clks@41300004 {

        compatible = "marvell,pxa-clocks";

        #clock-cells = <1>;

        status = "okay";

};