mailbox: imx: support RST channel

i.MX generic MU supports MU-A/B reset feature.

When stop/start remotecore, MU is not reset. So when Linux stop
remotecore, the MU-B side BCR may contain valid configuration,
because MU-B is not reset. So when linux start Mcore
again and notify Mcore, Mcore is not ready to handle MU interrupt
and cause issues. So need reset MU when stop Mcore.

Signed-off-by: Peng Fan <peng.fan@nxp.com>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>