ARM: imx: add suspend in ocram support for i.mx6sl

i.MX6SL's suspend in ocram function is derived from i.MX6Q,
it can lower the DDR IO power from ~10mA@1.2V to ~1mA@1.2V,
measured on i.MX6SL EVK board, SH5.

Signed-off-by: Anson Huang <b20788@freescale.com>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>