MAINTAINERS: Update Mun Yew Tham as Altera Mailbox Driver maintainer

Update Altera Mailbox Driver maintainer's email from <joyce.ooi@intel.com> to <mun.yew.tham@intel.com>

Signed-off-by: Mun Yew Tham <mun.yew.tham@intel.com>
Acked-by: Joyce Ooi <joyce.ooi@intel.com>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>