gpio: aggregator: move GPIO forwarder allocation in a dedicated function

gpio: aggregator: move GPIO forwarder allocation in a dedicated function

Move the GPIO forwarder allocation and static initialization in a dedicated
function.

Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Thomas Richard <thomas.richard@bootlin.com>
Link: https://lore.kernel.org/r/20250811-aaeon-up-board-pinctrl-support-v9-2-29f0cbbdfb30@bootlin.com
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>