gpio: rcar: Use irqchip template

This makes the driver use the irqchip template to assign
properties to the gpio_irq_chip instead of using the
explicit call to gpiochip_irqchip_add().

The irqchip is instead added while adding the gpiochip.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Cc: Biju Das <biju.das@bp.renesas.com>
Cc: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/20200722113141.243163-1-linus.walleij@linaro.org