ARM: imx: add suspend/resume support for i.mx6ul

This patch adds suspend function for i.MX6UL, it supports
"standby" and "mem" mode, for "standby" mode, SoC will
enter STOP mode only, while for "mem" mode, SoC will
enter STOP mode and DDR IO will be set to low power
mode.

As i.MX6UL contains a "Cortex-A7" ARM core which has no
PL310, so we need to avoid any PL310 operations during
suspend/resume, also, we need to flush Cortex-A7's inernal
L2 cache before suspend.

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