ARM: imx: correct low-power mode setting

The hardware reset value of bit CCM_CLPCR_LPM enables WAIT mode
(WAIT_UNCLOCKED) by default.  However this is undesirable because
WAIT mode should only be enabled when there is a driver managing
ARM clock gating.  Correct the initial power mode to WAIT_CLOCKED
(disable WAIT mode).  While at it, the power mode after resuming
is also set back to WAIT_CLOCKED from STOP_POWER_OFF.

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