bindings: ARM: add devicetree binding for describing PSCI firmware

* Power State Coordination Interface (PSCI)

Firmware implementing the PSCI functions described in ARM document number

ARM DEN 0022A ("Power State Coordination Interface System Software on ARM

processors") can be used by Linux to initiate various CPU-centric power

operations.

Issue A of the specification describes functions for CPU suspend, hotplug

and migration of secure software.

Functions are invoked by trapping to the privilege level of the PSCI

firmware (specified as part of the binding below) and passing arguments

in a manner similar to that specified by AAPCS:

	 r0		=> 32-bit Function ID / return value

	{r1 - r3}	=> Parameters

Note that the immediate field of the trapping instruction must be set

to #0.

Main node required properties:

 - compatible    : should contain at least one of:

     * "arm,psci"     : For implementations complying to PSCI versions prior

			to 0.2.

			For these cases function IDs must be provided.

     * "arm,psci-0.2" : For implementations complying to PSCI 0.2.

			Function IDs are not required and should be ignored by

			an OS with PSCI 0.2 support, but are permitted to be

			present for compatibility with existing software when

			"arm,psci" is later in the compatible list.

     * "arm,psci-1.0" : For implementations complying to PSCI 1.0.

			PSCI 1.0 is backward compatible with PSCI 0.2 with

			minor specification updates, as defined in the PSCI

			specification[2].

 - method        : The method of calling the PSCI firmware. Permitted

                   values are:

                   "smc" : SMC #0, with the register assignments specified

		           in this binding.

                   "hvc" : HVC #0, with the register assignments specified

		           in this binding.

Main node optional properties:

 - cpu_suspend   : Function ID for CPU_SUSPEND operation

 - cpu_off       : Function ID for CPU_OFF operation

 - cpu_on        : Function ID for CPU_ON operation

 - migrate       : Function ID for MIGRATE operation

Device tree nodes that require usage of PSCI CPU_SUSPEND function (ie idle

state nodes, as per bindings in [1]) must specify the following properties:

- arm,psci-suspend-param

		Usage: Required for state nodes[1] if the corresponding

                       idle-states node entry-method property is set

                       to "psci".

		Value type: <u32>

		Definition: power_state parameter to pass to the PSCI

			    suspend call.

Example:

Case 1: PSCI v0.1 only.

	psci {

		compatible	= "arm,psci";

		method		= "smc";

		cpu_suspend	= <0x95c10000>;

		cpu_off		= <0x95c10001>;

		cpu_on		= <0x95c10002>;

		migrate		= <0x95c10003>;

	};

Case 2: PSCI v0.2 only

	psci {

		compatible	= "arm,psci-0.2";

		method		= "smc";

	};

Case 3: PSCI v0.2 and PSCI v0.1.

	A DTB may provide IDs for use by kernels without PSCI 0.2 support,

	enabling firmware and hypervisors to support existing and new kernels.

	These IDs will be ignored by kernels with PSCI 0.2 support, which will

	use the standard PSCI 0.2 IDs exclusively.

	psci {

		compatible = "arm,psci-0.2", "arm,psci";

		method = "hvc";

		cpu_on = < arbitrary value >;

		cpu_off = < arbitrary value >;

		...

	};

[1] Kernel documentation - ARM idle states bindings

    Documentation/devicetree/bindings/arm/idle-states.txt

[2] Power State Coordination Interface (PSCI) specification

    http://infocenter.arm.com/help/topic/com.arm.doc.den0022c/DEN0022C_Power_State_Coordination_Interface.pdf