net: mdio: add helpers to extract clause 45 regad and devad fields

Add a couple of helpers and definitions to extract the clause 45 regad
and devad fields from the regnum passed into MDIO drivers.

Tested-by: Daniel Golle <daniel@makrotopia.org>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Signed-off-by: Daniel Golle <daniel@makrotopia.org>
Signed-off-by: David S. Miller <davem@davemloft.net>