gpio: lynxpoint: Convert to use gpiolib irqchip

Instead of open-coding irqchip handling in the driver we can take advantage
of the new irqchip helpers provided by the gpiolib core.

While doing this we also make sure that we call gpiochip_irqchip_add()
after the gpiochip itself is registered as required.

Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>