dt-bindings: Add documentation for rpm master stats

* RPM Stats

RPM maintains a counter of the masters i.e APPS, MPPS etc

number of times the SoC entered a deeper sleep mode involving

lowering or powering down the backbone rails - Cx and Mx and

the oscillator clock, XO.

PROPERTIES

- compatible:

	Usage: required

	Value type: <string>

	Definition: Should be "qcom,rpm-master-stats".

- reg:

	Usage: required

	Value type: <prop-encoded-array>

	Definition: The address on the RPM RAM from where the stats are read

	            should be provided as "phys_addr_base". The offset

		    from which the stats are available should be provided as

		    "offset_addr".

- reg-names:

	Usage: required

	Value type: <prop-encoded-array>

	Definition: Provides labels for the reg property.

- qcom,masters:

	Usage: required

	Value tye: <string list>

	Defination: Provides the masters list.

qcom,master-offset:

	Usage: required

	Value tye: <prop-encoded-array>

	Defination: Provides the masters list

EXAMPLE:

qcom,rpm-master-stats@60150 {

		compatible = "qcom,rpm-master-stats";

		reg = <0x45f0150 0x5000>;

		qcom,masters = "APSS", "MPSS", "ADSP", "CDSP", "TZ";

		qcom,master-stats-version = <2>;

		qcom,master-offset = <4096>;

	};