ARM: mach-artpec: add entry to MAINTAINERS

Signed-off-by: Lars Persson <larper@axis.com>
Signed-off-by: Jesper Nilsson <jesper.nilsson@axis.com>
Reviewed-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Olof Johansson <olof@lixom.net>