gpio: Rename devm_get_gpiod_from_child()

Rename devm_get_gpiod_from_child() into
devm_fwnode_get_gpiod_from_child() to reflect the fact that this
function is operating on a fwnode object.

Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Acked-by: Jacek Anaszewski <jacek.anaszewski@gmail.com>
Acked-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>