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>

Documentation/devicetree/bindings/arm/arm,scmi.txt

@@ -6,7 +6,7 @@ that are provided by the hardware platform it is running on, including power
 and performance functions.
 
 This binding is intended to define the interface the firmware implementing
-the SCMI as described in ARM document number ARM DUI 0922B ("ARM System Control
+the SCMI as described in ARM document number ARM DEN 0056A ("ARM System Control
 and Management Interface Platform Design Document")[0] provide for OSPM in
 the device tree.