Documentation: add DT bindings for ARM SCPI sensors

The System Control Processor (SCP) provides access to SoC sensors via
the System Control and Power Interface (SCPI) Message Protocol. Add
bindings to allow probing of these sensors. Also support referencing
of the sensors for setting up thermal zones via the thermal DT
bindings.

Signed-off-by: Punit Agrawal <punit.agrawal@arm.com>
Acked-by: Rob Herring <robh+dt@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Sudeep Holla <sudeep.holla@arm.com>