firmware: arm_scmi: add device power domain support using genpd

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

Cc: Kevin Hilman <khilman@baylibre.com>
Reviewed-by: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>