mailbox: imx: Clear GIEn bit at shutdown

GIEn is enabled at startup for RX doorbell mailboxes so
we need to clear the bit at shutdown in order to avoid
leaving the interrupt line enabled.

Signed-off-by: Daniel Baluta <daniel.baluta@nxp.com>
Reviewed-by: Oleksij Rempel <o.rempel@pengutronix.de>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>