dt-bindings: net: bcm4908-enet: include ethernet-controller.yaml

It should be /included/ by every Ethernet controller binding. It adds
support for various generic properties.

Suggested-by: Rob Herring <robh@kernel.org>
Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>