net: phy: marvell: refactor HWMON OOP style

Use a structure of Marvell PHY specific HWMON methods to reduce code
duplication. Store a pointer to this structure into the PHY driver's
driver_data member.
Signed-off-by: Marek Behún <kabel@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>