Docs: dt: add devicetree binding for describing arm64 SDEI firmware

* Software Delegated Exception Interface (SDEI)

Firmware implementing the SDEI functions described in ARM document number

ARM DEN 0054A ("Software Delegated Exception Interface") can be used by

Linux to receive notification of events such as those generated by

firmware-first error handling, or from an IRQ that has been promoted to

a firmware-assisted NMI.

The interface provides a number of API functions for registering callbacks

and enabling/disabling events. Functions are invoked by trapping to the

privilege level of the SDEI firmware (specified as part of the binding

below) and passing arguments in a manner specified by the "SMC Calling

Convention (ARM DEN 0028B):

	 r0		=> 32-bit Function ID / return value

	{r1 - r3}	=> Parameters

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

to #0.

The SDEI_EVENT_REGISTER function registers a callback in the kernel

text to handle the specified event number.

The sdei node should be a child node of '/firmware' and have required

properties:

 - compatible    : should contain:

	* "arm,sdei-1.0" : For implementations complying to SDEI version 1.x.

 - method        : The method of calling the SDEI 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.

Example:

	firmware {

		sdei {

			compatible	= "arm,sdei-1.0";

			method		= "smc";

		};

	};