drivers: nxp mbox imx mu driver fix Flags

This commit fixes flag handling in mbox driver.
There are two MU drivers mu/ and mu1/ in nxp sdk with
same API but different bit masks and register layouts.

To be compatible with both add mapping table for channel index.
This will use enum from relavant driver heder to select correct
flag mask.

Signed-off-by: Tomas Galbicka <tomas.galbicka@nxp.com>