We already have a possibility to register a MAC address provider
based on a ethernet device id. This adds a similar functionality
for devices probed from devicetree. Code can register itself to
be a MAC address provider for a certain devicetree node.

This helps on i.MX to let the IIM unit provide a MAC address for
the FEC.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>