firmware: arm_scmi: Move power-domain driver to the pmdomain dir

To simplify with maintenance let's move the Arm SCMI power-domain driver
to the new pmdomain directory.

Link: https://lore.kernel.org/all/20230921113328.3208651-1-sudeep.holla@arm.com
Reviewed-by: Ulf Hansson <ulf.hansson@linaro.org>
Cc: Cristian Marussi <cristian.marussi@arm.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>