gpio: of: Make of_get_named_gpiod_flags() private

Since commit f626d6df ("gpio: of: Break out OF-only code"),
there are no more users of of_get_named_gpiod_flags() outside
gpiolib-of.c.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/20190906084539.21838-2-geert+renesas@glider.be


Signed-off-by: Linus Walleij <linus.walleij@linaro.org>