sh-pfc: Make struct pinmux_gpio enum_id field const

This ensures that the field is not modified, which is a prerequisite for
the rest of the PFC refactoring work.

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