MAINTAINERS: Change mailbox-altera maintainer

Update the mailbox-alters maintainer from <mun.yew.tham@intel.com> to
<tiensung.ang@altera.com> as Tham mun yew is no longer with Altera.

Signed-off-by: Ang Tien Sung <tiensung.ang@altera.com>
Acked-by: Mun Yew Tham <mun.yew.tham@intel.com>
Signed-off-by: Jassi Brar <jassisinghbrar@gmail.com>

Author: Ang Tien Sung

MAINTAINERS

@@ -918,7 +918,7 @@ R:	Pali Rohár <pali@kernel.org>
 F:	drivers/input/mouse/alps.*
 
 ALTERA MAILBOX DRIVER
-M:	Mun Yew Tham <mun.yew.tham@intel.com>
+M:	Tien Sung Ang <tiensung.ang@altera.com>
 S:	Maintained
 F:	drivers/mailbox/mailbox-altera.c