dt-bindings: net: update bcmgenet binding for GENETv5

The device tree documentation must be updated to reflect the new compatible
strings "brcm,genet-v5" and "brcm,genet-mdio-v5" used by the GENETv5 driver.

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>