ARM: imx: enable WAIT mode hardware workaround for imx6sx

Need to enable INT_MEM_CLK_LPM bit in CCM_CGPR for WAIT mode,
without this bit set, if there is pending interrupt during
ARM platform entering WAIT mode without power gating, cache
data will be corrupted, this is a hardware workaround for WAIT
mode and must be enabled.

Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>