sh-pfc: Clean up pin configuration type handling

Set pin configuration type to

- PINMUX_TYPE_NONE at initialization time and when disabling a function
  or freeing a GPIO

- PINMUX_TYPE_FUNCTION when enabling a function

- PINMUX_TYPE_INPUT or PINMUX_TYPE_OUTPUT when setting the GPIO
  direction

Verify that the type is PINMUX_TYPE_NONE when enabling a function or
requesting a GPIO and return -EBUSY if it isn't.

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