net: axienet: document axistream-connected attribute

The axienet driver requires the use of an axistream-connected attribute,
but this isn't documented in the devicetree bindings. Document how this
attribute is supposed to be used, including the upcoming change to make
the usage of this attribute optional.

Signed-off-by: Robert Hancock <hancock@sedsystems.ca>
Signed-off-by: David S. Miller <davem@davemloft.net>