net: bcmgenet: communicate integrated PHY revision to PHY driver

The integrated BCM7xxx PHY contains no useful revision information in
its MII_PHYSID2 bits 3:0, that information is instead contained in the
GENET hardware block.

We already read the GENET 32-bit revision register, so store the
integrated PHY revision in the driver private structure, and then
communicate this revision value to the PHY driver by overriding the
phy_flags value.

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