dt-bindings: firmware: arm,scmi: Add support for powercap protocol

Add new SCMI v3.1 powercap protocol bindings definitions and example.

Link: https://lore.kernel.org/r/20220704102241.2988447-2-cristian.marussi@arm.com


Cc: Rob Herring <robh+dt@kernel.org>
Cc: devicetree@vger.kernel.org
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Cristian Marussi <cristian.marussi@arm.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>