net: phy: broadcom: Add statistics for all Gigabit PHYs

All Gigabit PHYs use the same register layout as far as fetching
statistics goes. Fast Ethernet PHYs do not all support statistics, and
the BCM54616S would require some switching between the coper and fiber
modes to fetch the appropriate statistics which is not supported yet.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>