ARM: imx6: disable the GIC CPU interface before calling stby-poweroff sequence

Any pending interrupt can prevent entering standby based power off state.
To avoid it, disable the GIC CPU interface.

Fixes: 8148d213 ("ARM: imx6: register pm_power_off handler if "fsl,pmic-stby-poweroff" is set")
Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>