Documentation: bindings: Add missing Amlogic SCPI sensor bindings

amlogic,meson-gxbb-scpi-sensors is both the driver and DT but is not
documented. Just add it to amlogic's scpi documentation
Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Kevin Hilman <khilman@baylibre.com>

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

@@ -17,4 +17,11 @@ Required sub-node properties:
 - compatible : should be "amlogic,meson-gxbb-scp-shmem" for SRAM based shared
 		memory on Amlogic GXBB SoC.
 
+Sensor bindings for the sensors based on SCPI Message Protocol
+--------------------------------------------------------------
+SCPI provides an API to access the various sensors on the SoC.
+
+Required properties:
+- compatible : should be "amlogic,meson-gxbb-scpi-sensors".
+
 [0] Documentation/devicetree/bindings/arm/arm,scpi.txt