Add a convenience macro for iterating over graph endpoints. Iterating over
graph endpoints using fwnode_graph_get_next_endpoint() is a recurring
pattern, and this macro allows calling that function in a slightly more
convenient way. For instance,

	for (child = NULL;
	     (child = fwnode_graph_get_next_endpoint(fwnode, child)); )

becomes

	fwnode_graph_for_each_endpoint(fwnode, child)
Signed-off-by: NSakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: NRafael J. Wysocki <rafael.j.wysocki@intel.com>