gpio: visconti: Fix fwnode of GPIO IRQ

The fwnode of GPIO IRQ must be set to its own fwnode, not the fwnode of the
parent IRQ. Therefore, this sets own fwnode instead of the parent IRQ fwnode to
GPIO IRQ's.

Fixes: 2ad74f40

 ("gpio: visconti: Add Toshiba Visconti GPIO support")
Signed-off-by: Nobuhiro Iwamatsu <nobuhiro1.iwamatsu@toshiba.co.jp>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Bartosz Golaszewski <brgl@bgdev.pl>