MAINTAINERS: add greybus subsystem mailing list

The Greybus driver subsystem has a mailing list, so list it in the
MAINTAINERS file so that people know to send patches there as well.

Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Acked-by: Johan Hovold <johan@kernel.org>
Reviewed-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>