drivers/gpio/nomadik: move dummy nmk_gpio_dbg_show_one() to header

drivers/gpio/nomadik: move dummy nmk_gpio_dbg_show_one() to header

When `CONFIG_DEBUG_FS` is disabled, nmk_gpio_dbg_show_one() is an
empty dummy function; this however triggers a `-Wmissing-prototypes`
warning and later a linker error because the function is also used by
drivers/pinctrl/nomadik/pinctrl-nomadik.c, therefore it needs to be
non-static.

To allow both sources to access this dummy function, this patch moves
it to the header, adding the `#ifdef CONFIG_DEBUG_FS` there as well.

Signed-off-by: Max Kellermann <max.kellermann@ionos.com>
Link: https://lore.kernel.org/r/20240311133223.3429428-1-max.kellermann@ionos.com
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>