ARM: imx6q: support WAIT mode using cpuidle

Add WAIT mode (ARM core clock gating) support to imx6q cpuidle driver.
As WAIT mode is broken on imx6q TO 1.0 and 1.1, it only enables the
support for revision 1.2 with chicken bit set.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>