Documentation: dt: add bindings for keystone pll control controller

* Device tree bindings for Texas Instruments keystone pll controller

The main pll controller used to drive theC66x CorePacs, the switch fabric,

and a majority of the peripheral clocks (all but the ARM CorePacs, DDR3 and

the NETCP modules) requires a PLL Controller to manage the various clock

divisions, gating, and synchronization.

Required properties:

- compatible:		"ti,keystone-pllctrl", "syscon"

- reg:			contains offset/length value for pll controller

			registers space.

Example:

pllctrl: pll-controller@0x02310000 {

	compatible = "ti,keystone-pllctrl", "syscon";

	reg = <0x02310000 0x200>;

};