firmware: scpi: add device power domain support using genpd

This patch hooks up the support for device power domain provided by
SCPI using the Linux generic power domain infrastructure.

Cc: "Rafael J. Wysocki" <rjw@rjwysocki.net>
Tested-by: Mathieu Poirier <mathieu.poirier@linaro.org>
Tested-by: Jon Medhurst <tixy@linaro.org>
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>