ARM: imx: add mem mode suspend for i.MX6SLL

Add mem mode suspend for i.MX6SLL, when linux
kernel suspend, SoC will enter STOP mode,
ARM core will be power gated and MMDC IO
will be set to low power mode.

Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>