firmware: arm_scpi: Extend to support sensors

ARM System Control Processor (SCP) provides an API to query and use
the sensors available in the system. Extend the SCPI driver to support
 sensor messages.

Signed-off-by: Punit Agrawal <punit.agrawal@arm.com>
Acked-by: Sudeep Holla <sudeep.holla@arm.com>