gpio: of: Allow overriding the device node

When registering a GPIO chip, drivers can override the device tree node
associated with the chip by setting the chip's ->of_node field. If set,
this field is supposed to take precedence over the ->parent->of_node
field, but the code doesn't actually do that.

Commit 762c2e46 ("gpio: of: remove of_gpiochip_and_xlate() and
struct gg_data") exposes this because it now no longer matches on the
GPIO chip's ->of_node field, but the GPIO device's ->of_node field that
is set using the procedure described above.

Signed-off-by: Thierry Reding <treding@nvidia.com>
Acked-by: Alexandre Courbot <acourbot@nvidia.com>
Reviewed-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>