pmdomain: arm_scmi: add support for domain hierarchies

After primary SCMI pmdomain is created, use new of_genpd helper which
checks for child domain mappings defined in power-domains-child-ids.

Also remove any child domain mappings when SCMI domain is removed.

Signed-off-by: Kevin Hilman (TI) <khilman@baylibre.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>