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 <[email protected]>
Signed-off-by: David S. Miller <[email protected]>

Author: elkablo