MAINTAINERS: Replace Ley Foon Tan as Altera Mailbox maintainer

This patch replaces Ley Foon Tan as Altera Mailbox maintainer as she has
moved to a different role.

Signed-off-by: Joyce Ooi <joyce.ooi@intel.com>
Acked-by: Ley Foon Tan <lftan.linux@gmail.com>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>