staging/fbtft: Depend on OF

staging/fbtft: Depend on OF

Commit c440eee1a7a1 ("Staging: fbtft: Switch to the gpio descriptor
interface") removed setting gpios via platform data. This means that
fbtft will now only work with Device Tree so set the dependency.

This also prevents a NULL pointer deref on non-DT platform because
fbtftops.request_gpios is not set in that case anymore.

Fixes: c440eee1a7a1 ("Staging: fbtft: Switch to the gpio descriptor interface")
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Link: https://lore.kernel.org/r/20190917171843.10334-1-noralf@tronnes.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>