net: phy: add Broadcom BCM84881 PHY driver

Add a rudimentary Clause 45 driver for the BCM84881 PHY, found on
Methode DM7052 SFPs.

Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>