fix(imx8m): fix the dram retention random hang on some imx8mq Rev2.0

fix(imx8m): fix the dram retention random hang on some imx8mq Rev2.0

It seems the DRAM APB clock root slice can NOT work normally
if the PLLs is power down in DSM mode. So update this clock
slice's setting explicitly to make it work. This piece of code
is there for a long while on previous release, so just add
it back to align with previous flow.

Signed-off-by: Jacky Bai <ping.bai@nxp.com>
Reviewed-by: Anson Huang <Anson.Huang@nxp.com>
Change-Id: I113069494074194e116fdb1229052d2956bf90ea