staging: greybus: move es2 to drivers/greybus/

staging: greybus: move es2 to drivers/greybus/

The es2 Greybus host controller has long been stable, so move it out of
drivers/staging/ to drivers/greybus/

Cc: Johan Hovold <johan@kernel.org>
Cc: greybus-dev@lists.linaro.org
Cc: linux-kernel@vger.kernel.org
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Acked-by: Alex Elder <elder@kernel.org>
Link: https://lore.kernel.org/r/20190825055429.18547-10-gregkh@linuxfoundation.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>