Documentation: bindings: Add support for Amlogic GXBB SCPI protocol

Acked-by: NRob Herring <robh@kernel.org>
Signed-off-by: NNeil Armstrong <narmstrong@baylibre.com>
(decoupled from the generic scpi binding)
Signed-off-by: NSudeep Holla <sudeep.holla@arm.com>

Documentation/devicetree/bindings/arm/amlogic,scpi.txt

+System Control and Power Interface (SCPI) Message Protocol
+(in addition to the standard binding in [0])
+----------------------------------------------------------
+Required properties
+
+- compatible : should be "amlogic,meson-gxbb-scpi"
+
+AMLOGIC SRAM and Shared Memory for SCPI
+------------------------------------
+
+Required properties:
+- compatible : should be "amlogic,meson-gxbb-sram"
+
+Each sub-node represents the reserved area for SCPI.
+
+Required sub-node properties:
+- compatible : should be "amlogic,meson-gxbb-scp-shmem" for SRAM based shared
+		memory on Amlogic GXBB SoC.
+
+[0] Documentation/devicetree/bindings/arm/arm,scpi.txt