dt-bindings: arm: fix the document ID for SCMI protocol documentation

arm,scmi.txt used the wrong document identifier. "ARM DUI 0922B" is
the "ARM Compute Subsystem SCP, Message Interface Protocols". What we
need is the ARM DEN 0056A - "ARM System Control and Management
Interface Platform Design Document".

Fixes: fe7be8b2 ("dt-bindings: arm: add support for ARM System Control and Management Interface(SCMI) protocol")
Signed-off-by: Volodymyr Babchuk <volodymyr_babchuk@epam.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>