staging: greybus: move the greybus core to drivers/greybus

The Greybus core code has been stable for a long time, and has been
shipping for many years in millions of phones. With the advent of a
recent Google Summer of Code project, and a number of new devices in the
works from various companies, it is time to get the core greybus code
out of staging as it really is going to be with us for a while.

Cc: Johan Hovold <johan@kernel.org>
Cc: linux-kernel@vger.kernel.org
Cc: greybus-dev@lists.linaro.org
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Acked-by: Alex Elder <elder@kernel.org>
Link: https://lore.kernel.org/r/20190825055429.18547-9-gregkh@linuxfoundation.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>