ARM: imx: Add imx6q cpuidle driver

Add basic imx6q cpuidle driver.  For now, only basic WFI state is
supported.  Deeper idle states will be added in the future.

Signed-off-by: Robert Lee <rob.lee@linaro.org>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>