device property: Implement fwnode_graph_get_endpoint_count()

Add fwnode_graph_get_endpoint_count() function to provide generic
implementation of of_graph_get_endpoint_count(). The former by default
only counts endpoints to available devices which is consistent with the
rest of the fwnode graph API. By providing FWNODE_GRAPH_DEVICE_DISABLED
flag, also unconnected endpoints and endpoints to disabled devices are
counted.

Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>