arm: omap1: ams-delta: stop using gpiochip_find()

gpiochip_find() is going away as it's not hot-unplug safe. This platform
is not affected by any of the related problems as this GPIO controller
cannot really go away but in order to finally remove this function, we
need to convert it to using gpio_device_find() as well.

Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Janusz Krzysztofik <jmkrzyszt@gmail.com>
Acked-by: Tony Lindgren <tony@atomide.com>