docs: driver-api: bt8xxgpio: Revive dead link

www.bu3sch.de has been unusable for several years, but the same
information is available on bues.ch.

Signed-off-by: Jonathan Neuschäfer <j.neuschaefer@gmx.net>
Link: https://lore.kernel.org/r/20191004164059.10397-2-j.neuschaefer@gmx.net
Acked-by: Michael Büsch <m@bues.ch>
Reviewed-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>