gpiolib: fix chip order in gpio list

In some situations the gpio_list order is not correct.
As a consequence gpiochip_find_base returns the same
base number twice. This happens when a first ship is added
with manual base number, then other ships are added using
automatic base number.

To prevent this behaviour, this patch add the new chip after
the last element of the gpio list.

Signed-off-by: Julien Grossholtz <julien.grossholtz@savoirfairelinux.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>