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

* Time Base Generator Clock bindings for Marvell Armada 37xx SoCs

Marvell Armada 37xx SoCs provde Time Base Generator clocks which are

used as parent clocks for the peripheral clocks.

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

clock ID in its "clocks" phandle cell.

The following is a list of provided IDs and clock names on Armada 3700:

 0 = TBG A P

 1 = TBG B P

 2 = TBG A S

 3 = TBG B S

Required properties:

- compatible : shall be "marvell,armada-3700-tbg-clock"

- reg : must be the register address of North Bridge PLL register

- #clock-cells : from common clock binding; shall be set to 1

Example:

tbg: tbg@13200 {

	compatible = "marvell,armada-3700-tbg-clock";

	reg = <0x13200 0x1000>;

	clocks = <&xtalclk>;

	#clock-cells = <1>;

};