staging: Move ralink-gdma to its own directory

This is in preparation to allow it and the mt7621-dma drivers to be
built separately.  They are completely independent pieces of software,
and the Kconfig specifies very different requirements.

Cc: linux-kernel@vger.kernel.org
Cc: devel@driverdev.osuosl.org
Cc: Neil Brown <neil@brown.name>
Signed-off-by: George Hilliard <thirtythreeforty@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>