staging: greybus: add some MAINTAINERS

Johan and Alex and I are going to maintain the greybus code, so add it
to MAINTAINERS so we get cc:ed on patches.

Acked-by: Johan Hovold <johan@kernel.org>
Acked-by: Alex Elder <elder@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>