dt-bindings: net: document bcmgenet WoL interrupt

A third interrupt cell can be provided to optionally specify
the interrupt used for handling Wake on LAN events.

Typically the wake up handling uses a separate interrupt
controller, so the interrupts-extended property is used to
accommodate this.

Signed-off-by: Doug Berger <opendmb@gmail.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>