net: dsa: mv88e6xxx: prefix PHY macros

Prefix the PHY_* macros with a Marvell specific MV88E6XXX_ prefix.
There is no functional changes.

Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Signed-off-by: David S. Miller <davem@davemloft.net>