gpio: of: Scan available child node for gpio-hog

Look for child node which are available when iterating for
gpio hog node for request/set GPIO initial configuration
during OF gpio chip registration.

All it really does is make it possible to set
status = "disabled"; in the hog nodes, and then they will
not be applied.

Signed-off-by: Laxman Dewangan <ldewangan@nvidia.com>
Reviewed-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>