dt-bindings: Add ddr_stats documentation

* DDR Stats

AOP maintains DDR statistics like DDR LPMs and frequency stats. DDR stats

driver gives sysfs interface to display this.

PROPERTIES

- compatible:

	Usage: required

	Value type: <string>

	Definition: Should be "qcom,ddr-stats".

- reg:

	Usage: required

	Value type: <prop-encoded-array>

	Definition: The address on the AOP Message 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.

EXAMPLE:

	qcom,ddr-stats@c300000 {

		compatible = "qcom,ddr-stats";

		reg = <0xc300000 0x1000>, <0xc3f001c 0x4>;

		reg-names = "phys_addr_base", "offset_addr";

	};