gpio: rcar: Switch to use gpiolib irqchip helpers

Switch the R-Car Gen2 GPIO driver to use the gpiolib irqchip helpers.

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

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>