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

The Software Delegated Exception Interface (SDEI) is an ARM standard
for registering callbacks from the platform firmware into the OS.
This is typically used to implement RAS notifications, or from an
IRQ that has been promoted to a firmware-assisted NMI.

Add a new devicetree binding to describe the SDE firmware interface.

Signed-off-by: James Morse <james.morse@arm.com>
Acked-by: Rob Herring <robh@kernel.org>

Changes since v2:
 * Added Rob's Ack
 * Fixed 'childe node' typo

Changes since v1:
* Added bound IRQ description for binding,
* Reference SMC-CC, not 'AAPCS like'
* Move sdei node under firmware node (and the file path)