gpiolib: make legacy interfaces optional

The traditional interfaces are only used on a small number of ancient
boards. Make these optional now so they can be disabled by default.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Alexander Sverdlin <alexander.sverdlin@gmail.com>
Link: https://lore.kernel.org/r/20250722153634.3683927-1-arnd@kernel.org


Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>