net: dsa: mv88e6xxx: Move phy functions into phy.[ch]

The upcoming SERDES support will need to make use of PHY functions. Move
them out into a file of there own. No code changes.

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