dt-bindings: net: bcmgenet: Document 7712 binding

7712 includes a GENETv5 adapter with an on-chip 10/100 16nm Ethernet PHY
which requires us to document that controller's integration specifically
for proper driver keying.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>