gpio: rcar: Add support for DT IRQ flags

The gpio-rcar driver has no IRQ domain OF xlate function and thus
ignores IRQ flags specified in DT. Fix this by using the two-cell xlate
function.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>