gpiolib: use gpio_chips list in gpiochip_find

Using the GPIO chips list is much faster than parsing the entire GPIO
number space.

Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>