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>

MAINTAINERS

@@ -5504,6 +5504,7 @@ M:	Alex Elder <elder@kernel.org>
 M:	Greg Kroah-Hartman <gregkh@linuxfoundation.org>
 S:	Maintained
 F:	drivers/staging/greybus/
+L:	greybus-dev@lists.linaro.org
 
 GREYBUS AUDIO PROTOCOLS DRIVERS
 M:	Vaibhav Agarwal <vaibhav.sr@gmail.com>