dt-bindings: net: bcm4908-enet: fix Ethernet generic properties

This binding file uses $ref: ethernet-controller.yaml# so it's required
to use "unevaluatedProperties" (instead of "additionalProperties") to
make Ethernet properties validate.

Fixes: f08b5cf1

 ("dt-bindings: net: bcm4908-enet: include ethernet-controller.yaml")
Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Signed-off-by: David S. Miller <davem@davemloft.net>