All function GPIO entries are initialized with the GPIO_FN macro that
expands to the PINMUX_GPIO macro, used to initialize real GPIOs. Create
a PINMUX_GPIO_FN macro that duplicates PINMUX_GPIO and sets flags to
PINMUX_TYPE_FUNCTION and use it in GPIO_FN, and make PINMUX_GPIO set
flags to PINMUX_TYPE_GPIO.

This removes the need to initialize GPIO flags at runtime and thus
simplifies the code, preparing for the GPIO and functions split.
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>