imx: imx8qm/imx8qxp: Power down the resources before SPL jump to u-boot

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>