media: software_node: Add support for fwnode_graph*() family of functions

This implements the remaining .graph_*() callbacks in the fwnode
operations structure for the software nodes. That makes the
fwnode_graph_*() functions available in the drivers also when software
nodes are used.

The implementation tries to mimic the "OF graph" as much as possible, but
there is no support for the "reg" device property. The ports will need to
have the index in their  name which starts with "port@" (for example
"port@0", "port@1", ...) and endpoints will use the index of the software
node that is given to them during creation. The port nodes can also be
grouped under a specially named "ports" subnode, just like in DT, if
necessary.

The remote-endpoints are reference properties under the endpoint nodes
that are named "remote-endpoint".

Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: Heikki Krogerus <heikki.krogerus@linux.intel.com>
Co-developed-by: Daniel Scally <djrscally@gmail.com>
Signed-off-by: Daniel Scally <djrscally@gmail.com>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Acked-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>