net: phy: bcm7xxx: add support for 28nm EPHY

This commit adds support for the internal fast ethernet 10/100 PHY
found in the BCM7260, BCM7268, and BCM7271 devices.

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>