net: dsa: mv88e6060: replace magic values with register defines

To align with the mv88e6xxx code, use the register defines to
access all the register addresses and bit fields.

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