net: dsa: mv88e6xxx: Allow PCS registers to be retrieved via ethtool

ethtool provides a generic mechanism for a driver to return the
registers of an ethernet device. DSA uses this to give the port
registers associated with an interfaces. Extend this to allow PCS
registers to also be returned, if the port has a PCS associated to it.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>