gpio: aggregator: handle runtime registration of gpio_desc in gpiochip_fwd

gpio: aggregator: handle runtime registration of gpio_desc in gpiochip_fwd

Add request() callback to check if the GPIO descriptor was well registered
in the gpiochip_fwd before using it. This is done to handle the case where
GPIO descriptor is added at runtime in the forwarder.

If at least one GPIO descriptor was not added before the forwarder
registration, we assume the forwarder can sleep as if a GPIO is added at
runtime it may sleep.

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-v9-7-29f0cbbdfb30@bootlin.com
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>