MAINTAINERS: change the status of vexpress board to Orphan

The email address of Matt Waddel is no longer working.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Cc: Marc Zyngier <marc.zyngier@arm.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Alexei Fedorov <alexie.fedorov@arm.com>