MLK-10891 ARM: imx: improve ddr exit flow when dsm enter fail on i.mx7d

On i.MX7D, when Mega/Fast MIX is on and system enter DSM fail,
DDR controller no need to do the retention exit flow, but just
exit self-refresh is enough.

Improve the arm register usage to make asm code easy to
read as well.

Signed-off-by: Anson Huang <b20788@freescale.com>