gpio.h: fix location of gpio legacy documentation

The location of this doc file was moved. Change its reference
accordingly.

Fixes: 7ee2c130

 ("Documentation: gpio: Move legacy documentation to driver-api")
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>