dt-bindings: Add documentation for system_pm_rpm

SYSTEM PM

System PM device is a virtual device that handles all CPU subsystem low power

mode activties. When entering core shutdown, resource state that were requested

from the processor may be relinquished and set to idle and restored when the

cores are brought out of sleep.

PROPERTIES

- compatible:

	Usage: required

	Value type: <string>

	Definition: must be "qcom,system-pm-rpm".

EXAMPLE

	system_pm_rpm {

		compatible = "qcom,system-pm-rpm";

	};