gpiolib: Extend software-node support to support secondary software-nodes

gpiolib: Extend software-node support to support secondary software-nodes

When a software-node gets added to a device which already has another
fwnode as primary node it will become the secondary fwnode for that
device.

Currently if a software-node with GPIO properties ends up as the secondary
fwnode then gpiod_find_by_fwnode() will fail to find the GPIOs.

Add a new gpiod_fwnode_lookup() helper which falls back to calling
gpiod_find_by_fwnode() with the secondary fwnode if the GPIO was not
found in the primary fwnode.

Fixes: e7f9ff5dc90c ("gpiolib: add support for software nodes")
Cc: stable@vger.kernel.org
Signed-off-by: Hans de Goede <hansg@kernel.org>
Reviewed-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Link: https://lore.kernel.org/r/20250920200955.20403-1-hansg@kernel.org
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>