Keep A53 PLAT(SCU) power domain on in wait mode(ret).
RBC count only need to be set in PLAT OFF mode, so
change it accordingly.

Signed-off-by: Jacky Bai <ping.bai@nxp.com>
Change-Id: Ie55e25c8210d298506fc4dca7a9653583db45e0c

for the i.MX8M SOCs, part of the code for gpc
and PSCI implementation can be reused and make it
common for all these SoCs. this patch extracts
the common part for reuse.

Signed-off-by: Jacky Bai <ping.bai@nxp.com>