dt-bindings: Add rpm-smd documentation

Resource Power Manager(RPM)

RPM is a dedicated hardware engine for managing shared SoC resources,

which includes buses, clocks, power rails, etc.  The goal of RPM is

to achieve the maximum power savings while satisfying the SoC's

operational and performance requirements.  RPM accepts resource

requests from multiple RPM masters.  It arbitrates and aggregates

the requests, and configures the shared resources. The RPM masters

are the application processor, the modem processor, as well as hardware

accelerators. The RPM driver communicates with the hardware engine using

SMD.

The devicetree representation of the RPM block should be:

Required properties

- compatible: "qcom,rpm-smd"

- rpm-channel-name: The string corresponding to the channel name of the

			peripheral subsystem. Required for both smd and

			glink transports.

- rpm-channel-type: The interal SMD edge for this subsystem found in

			<soc/qcom/smd.h>

- interrupts: The IRQ used by remote processor to inform APSS about

				reception of response message packet.

Optional properties

- rpm-standalone: Allow RPM driver to run in standalone mode irrespective of RPM

			channel presence.

- reg: Contains the memory address at which rpm messaging format version is

  stored. If this field is not present, the target only supports v0 format.

Example:

	qcom,rpm-smd {

		compatible = "qcom,rpm-smd";

		interrupts = <GIC_SPI 194 IRQ_TYPE_EDGE_RISING>;

		qcom,rpm-channel-name = "rpm_requests";

		qcom,rpm-channel-type = 15; /* APPS_RPM_SMD */

	}

}