plat: imx8mq: refact the dram low power code

refact the dram low power related code to make it more
friendly for different dram config or different board.

Signed-off-by: Bai Ping <ping.bai@nxp.com>
(cherry picked from commit c290a9e1664628a13a5e75e2400c5cc17882fbba)

Conflicts:
	plat/imx/imx8mq/sip_svc.c
12 files changed