Split the GPIOs table into a pins table for real GPIOs and a functions
table for function GPIOs.

Only register pins with the pinctrl core. The function GPIOs remain
accessible as GPIOs.
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>