device property: Check fwnode->secondary in fwnode_graph_get_next_endpoint()

Sensor drivers often check for an endpoint to make sure that they're
connected to a consuming device like a CIO2 during .probe(). Some of
those endpoints might be in the form of software_nodes assigned as
a secondary to the device's fwnode_handle. Account for this possibility
in fwnode_graph_get_next_endpoint() to avoid having to do it in the
sensor drivers themselves.

Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Daniel Scally <djrscally@gmail.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>