gpio: rcar: set IRQ chip parent_device

This enables Runtime PM handling for interrupts.

By setting the parent_device in struct irq_chip genirq will call the
pm_runtime_get/put APIs when an IRQ is requested/freed.

Signed-off-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>