dt: bindings: ux500: Add documentation for PM domains

* ST-Ericsson UX500 PM Domains

UX500 supports multiple PM domains which are used to gate power to one or

more peripherals on the SOC.

The implementation of PM domains for UX500 are based upon the generic PM domain

and use the corresponding DT bindings.

==PM domain providers==

Required properties:

 - compatible: Must be "stericsson,ux500-pm-domains".

 - #power-domain-cells : Number of cells in a power domain specifier, must be 1.

Example:

	pm_domains: pm_domains0 {

		compatible = "stericsson,ux500-pm-domains";

		#power-domain-cells = <1>;

	};

==PM domain consumers==

Required properties:

 - power-domains: A phandle and PM domain specifier. Below are the list of

		valid specifiers:

		Index	Specifier

		-----	---------

		0	DOMAIN_VAPE

Example:

	sdi0_per1@80126000 {

		compatible = "arm,pl18x", "arm,primecell";

		power-domains = <&pm_domains DOMAIN_VAPE>

	};