net: bcmgenet: add support for new GENET PHY revision scheme

Starting with GPHY revision G0, the GENET register layout has changed to
use the same numbering scheme as the Starfighter 2 switch. This means
that GPHY major revision is in bits 15:12, minor in bits 11:8 and patch
level is in bits 7:4.

Introduce a small heuristic which checks for the old scheme first, tests
for the new scheme and finally attempts to catch reserved values and
aborts.

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