mailbox: imx: fix RST channel support

Because IMX_MU_xCR_MAX was increased to 5, some mu cfgs were not updated
to include the CR register. Add the missed CR register to xcr array.

Fixes: 82ab513b ("mailbox: imx: support RST channel")
Reported-by: Liu Ying <victor.liu@nxp.com>
Signed-off-by: Peng Fan <peng.fan@nxp.com>
Tested-by: Liu Ying <victor.liu@nxp.com> # i.MX8qm/qxp MEK boards boot
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>