IPU7: FIX GMSL + MIPI

[syafiqnizam25]

This pull request refactors and improves the device registration and subdevice handling logic in ipu7-isys.c, mainly to better support both platform-data and device-tree based configurations. It introduces a helper function to robustly fetch fwnode endpoints, adjusts the order and conditions for registering external subdevices, and ensures proper cleanup. The most important changes are:

Device Tree / fwnode Handling Improvements

• Added the isys_get_fwnode_endpoint_by_id helper function to recursively fetch the correct fwnode endpoint, improving robustness in cases where endpoints may be attached to secondary fwnodes. (isys_get_fwnode_endpoint_by_id, ipu7-isys.c)
• Updated endpoint fetching in isys_notifier_init to use the new helper function, ensuring endpoints are found even in more complex fwnode hierarchies. (isys_notifier_init, ipu7-isys.c)

Subdevice Registration Logic

• Changed the order and condition for registering external subdevices in isys_register_devices, so that they are always registered if platform data is present, and the notifier is always initialized. Also, subdev nodes are registered only if there is no platform data or the notifier's waiting list is empty. (isys_register_devices, ipu7-isys.c)
• In isys_register_ext_subdev, added logic to unregister subdevices that lack fwnodes, ensuring proper cleanup for manually linked platform-data subdevs. (isys_register_ext_subdev, ipu7-isys.c)

Cleanup

• Simplified the device removal logic in isys_remove by always cleaning up the notifier, regardless of whether platform data is present. (isys_remove, ipu7-isys.c)

• tested with isx031 gmsl + mipi