ARM: imx6: cpuidle: omit the unnecessary unmask of GINT

In imx6_set_lpm, we only need to unmask GINT when not WAIT_CLOCKED,
so add a check condition.

Signed-off-by: Peng Fan <peng.fan@nxp.com>
Signed-off-by: Kohji Okuno <okuno.kohji@jp.panasonic.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>