gpio: regmap: set gpio_chip of_node

This is needed for properly registering GPIO regmap as a child of a regmap
pin controller.

Signed-off-by: Álvaro Fernández Rojas <noltari@gmail.com>
Reviewed-by: Michael Walle <michael@walle.cc>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Acked-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Link: https://lore.kernel.org/r/20210324081923.20379-3-noltari@gmail.com


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