MLK-14585-1 power: domain: expose generic_pm_domain structure to clients

MLK-14585-1 power: domain: expose generic_pm_domain structure to clients

In some platforms, accessing registers needs to make sure
power domain is enabled, such as for clock operations, power
domain needs to be enabled first before accessing clock
registers, so some clocks need to know its power domain's
status, it will need to get power domain structure by phandle,
expose the API to support this case.

Signed-off-by: Anson Huang <Anson.Huang@nxp.com>