mailbox: imx-mailbox: support i.MX8ULP MU

i.MX8ULP MU has different register layout and bit layout compared
with i.MX6SX/7ULP/8.

So add enum imx_mu_type to show it is IMX_MU_V2 or IMX_MU_V1.

For IMX_MU_V2 mu hardware, check it when calculating bit offset to get
the correct offset.

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