gpio: adnp: switch to use irqchip helpers

This switches the ADNP GPIO driver to use the gpiolib
irqchip helpers. Also do some random refactoring to make it
look like most other GPIO drivers.

Cc: Roland Stigge <stigge@antcom.de>
Cc: Lars Poeschel <poeschel@lemonage.de>
Cc: Thierry Reding <thierry.reding@gmail.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>