gpiolib / ACPI: document the GPIO descriptor based interface

In addition to the existing ACPI specific GPIO interface, document the new
descriptor based GPIO interface in Documentation/acpi/enumeration.txt, so
it is clear that this new interface is preferred over the ACPI specific
version.

Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Acked-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Reviewed-by: Alexandre Courbot <acourbot@nvidia.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>