net: dsa: mv88e6xxx: Add 6390 family PCS registers to ethtool -d

The mv88e6390 has upto 8 sets of PCS registers, depending on how ports
9 and 10 are configured. The can be spread over 8 ports. If a port has
a PCS register set, return it along with the port registers. The
register space is sparse, so hard code a list of registers which will
be returned. It can later be extended, if needed, by append to the end
of the list.

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