gpio / ACPI: return -ENOENT when no mapping exists

Doing this allows drivers to distinguish between a real error case (if
there was an error when we tried to resolve the GPIO) and when the optional
GPIO line was not available.

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