Documentation: add DT bindings for ARM SCPI power domains

The System Control Processor (SCP) provides peripheral devices with
power domains that can be enabled and disabled viathe System Control
and Power Interface (SCPI) Message Protocol. Add bindings to allow
probing of these device power domians.

Cc: Rob Herring <robh+dt@kernel.org>
Tested-by: Mathieu Poirier <mathieu.poirier@linaro.org>
Tested-by: Jon Medhurst <tixy@linaro.org>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Reviewed-by: Jon Medhurst <tixy@linaro.org>
Reviewed-by: Kevin Hilman <khilman@baylibre.com>
Reviewed-by: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>