dt-bindings: firmware: arm,scmi: Add support for system power protocol

Add SCMI System Power Protocol bindings, and the protocol id is 0x12.

Signed-off-by: Peng Fan <peng.fan@nxp.com>
Reviewed-by: Rob Herring (Arm) <robh@kernel.org>
Message-Id: <20240628030309.1162012-1-peng.fan@oss.nxp.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>