net: bcmgenet: add support for the GENETv5 hardware

This commit adds support for the GENETv5 implementation.

The GENETv5 reports a major version of 6 instead of 5 so compensate
for this when verifying the configuration of the driver.  Also the
EPHY revision is now contained in the MDIO registers of the PHY so
the EPHY revision of 0 in GENET_VER_FMT is expected for GENETv5.

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>