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 <[e-mail]>
Reviewed-by: Rob Herring <[e-mail]>
Signed-off-by: Kevin Hilman <[e-mail]>

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

···
 - 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