Documentation / ACPI: Fix location of GPIO documentation

Commit fd8e198cfcaa ("Documentation: gpiolib: document new interface")
moved Documentation/gpio.txt to Documentation/gpio/gpio-legacy.txt and added
new documents for descriptor-based interface so fix the the location here to
point Documentation/gpio/ since that what commit ccb6fbb99020
("Documentation / ACPI: update to GPIO descriptor API") was looking for.

Signed-off-by: Jarkko Nikula <jarkko.nikula@linux.intel.com>
Acked-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>