Revert "at86rf230: convert to gpio descriptors"

This reverts commit 622bd6ea.

Dmitry Torokhov points out that this conversion leaves an existing board
in reset state due to not properly handled polarity. Additionally, the
GPIO name inadvertenly changes from "reset-gpio" to "rstn-gpios".
Revert to avoid these regressions.

Follow up patches for a better conversion are applied as well.

Signed-off-by: Stefan Schmidt <stefan@datenfreihafen.org>