gpio: of: Make of_gpio_simple_xlate() private

Since commit 9a95e8d2 ("gpio: remove etraxfs driver"), there are
no more users of of_gpio_simple_xlate() outside gpiolib-of.c.
All GPIO drivers that need it now rely on of_gpiochip_add() setting it
up as the default translate function.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/20190906084539.21838-3-geert+renesas@glider.be


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