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>