gpio: of: Extract of_gpiochip_add_hog()

Extract the code to add all GPIO hogs of a gpio-hog node into its own
function, so it can be reused.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/20200220130149.26283-2-geert+renesas@glider.be


Reviewed-by: Frank Rowand <frank.rowand@sony.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>