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: NMathieu Poirier <mathieu.poirier@linaro.org>
Tested-by: NJon Medhurst <tixy@linaro.org>
Acked-by: NMark Rutland <mark.rutland@arm.com>
Reviewed-by: NJon Medhurst <tixy@linaro.org>
Reviewed-by: NKevin Hilman <khilman@baylibre.com>
Reviewed-by: NUlf Hansson <ulf.hansson@linaro.org>
Signed-off-by: NSudeep Holla <sudeep.holla@arm.com>