feat(imx8ulp): add system power off support

On i.MX8ULP, we need to use the APD deep power down(DPD) mode
to support the system power off function. when APD enter
power off mode, only the RTD can re-kick it and boot from ROM.

Signed-off-by: Jacky Bai <ping.bai@nxp.com>
Reviewed-by: Ye Li <ye.li@nxp.com>
Change-Id: Ifb42db0a7cf87b932160c59b47eca4d0f08f8cdf
1 file changed