net: dsa: mv88e6060: add register defines header file

To align with the mv88e6xxx code, add a similar header file
with all the register defines.
The file is based on the mv88e6xxx header for coherency.

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