Make sure that all devices that are powered up by SPL are powered down
before entering into the u-boot. Otherwise the subsystem/device will
never be powered down by SCFW, due to SPL and u-boot are in different
partitions.
Benefiting from power domain driver, this patch implements the function
"imx8_power_off_pd_devices" to power off all active devices.
Signed-off-by: Peng Fan <peng.fan@nxp.com>