gpio: rcar: reference device instead of platform device

The change simplifies dereferences to the mediated struct device, also
it allows to limit the scope of the platform device usage to probe and
remove functions only.

Non-functional change.

Signed-off-by: Vladimir Zapolskiy <vz@mleia.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>