sh-pfc: Use pinmux identifiers in the pin muxing API

The PFC core exposes a sh_pfc_config_gpio() function that configures
pinmuxing for a given GPIO (either a real GPIO or a function GPIO).
Handling of real and function GPIOs belong to the GPIO layer, move the
GPIO number to mark translation to the caller and rename the function to
sh_pfc_config_mux().

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