mailbox: add i.MX Messaging Unit (MU) driver

This patch provides a driver for i.MX Messaging Unit (MU) using the
commom mailbox framework.

This is ported from Linux (v6.12.8) driver
drivers/mailbox/imx-mailbox.c. Its commit SHA is:
39d7d6177f0c ("mailbox: imx: use device name in interrupt name")

Signed-off-by: Viorel Suman <viorel.suman@nxp.com>
Signed-off-by: Peng Fan <peng.fan@nxp.com>
Signed-off-by: Alice Guo <alice.guo@nxp.com>
Reviewed-by: Ye Li <ye.li@nxp.com>
diff --git a/MAINTAINERS b/MAINTAINERS
index e2339ec..92d4a15 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -315,6 +315,7 @@
 F:	board/freescale/common/
 F:	common/spl/spl_imx_container.c
 F:	doc/imx/
+F:	drivers/mailbox/imx-mailbox.c
 F:	drivers/serial/serial_mxc.c
 F:	include/imx_container.h