ARM: imx: add standby mode support for suspend

Add standby mode support for suspend, to enter standby mode:

echo standby > /sys/power/state;

Use UART or RTC alarm to wake up system, when system enters
standby mode, SOC will enter STOP mode with ARM core kept
power on and 24M XTAL on.

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