dsa: add support for the Marvell 88E6060 switch chip

Add support for the Marvell 88E6060 switch chip.  This chip only
supports the Header and Trailer tagging formats, and we use it in
Trailer mode since that mode is slightly easier to handle than
Header mode.

Signed-off-by: Lennert Buytenhek <buytenh@marvell.com>
Tested-by: Byron Bradley <byron.bbradley@gmail.com>
Tested-by: Tim Ellis <tim.ellis@mac.com>
Signed-off-by: David S. Miller <davem@davemloft.net>