MAINTAINERS: greybus-dev list is members-only

The greybus-dev mailing list is a members-only list and is
moderated for non-subscribers.

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Acked-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>