Also I agree that attempting to make this optional property/subnode
generic to all of CAN would be preferable. Another not sure if its
feasible yet without standardization being first forced across all CAN
drivers.

It should be. All you need to do is add an
of_get_can_maxspeed(struct device_node *np)
for drivers to call.

If it finds the property, return its value, otherwise return the
standardised 1Mbps.

And you probably want to do the verify in can_changelink(), by adding
the max speed into the can_priv structure somewhere.

    Andrew