Because that the mailbox MU driver is used in i.MX RPMSG implementation.
There is a confliction between this MU driver and the mailbox MU driver.
To back-compaible with LPM of iMX6SX, iMX7D and iMX7ULP.
Rename the compatible node of this MU driver.
Signed-off-by: Richard Zhu <hongxing.zhu@nxp.com>