gpio: ath79: Pass irqchip when adding gpiochip

We need to convert all old gpio irqchips to pass the irqchip
setup along when adding the gpio_chip.

For chained irqchips this is a pretty straight-forward
conversion.

Take this opportunity to add a local dev pointer and
use devm_gpiochip_add() so we can get rid of the remove()
callback altogether.

Cc: linux-mips@linux-mips.org
Acked-by: Alban Bedel <albeu@free.fr>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>