device property: Add a note to the fwnode.h

device property: Add a note to the fwnode.h

Add a note to the fwnode.h that the header should not be used
directly in the leaf drivers, they all should use the higher
level APIs and the respective headers.

The purpose of this note is to give guidance to driver writers
to avoid repeating a common mistake.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Reviewed-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Reviewed-by: Zijun Hu <quic_zijuhu@quicinc.com>
Link: https://lore.kernel.org/r/20250408095229.1298005-1-andriy.shevchenko@linux.intel.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>