dsa: mv88e6xxx: Set the RGMII delay based on phy interface

Some Marvell switches allow the RGMII Rx and Tx clock to be delayed
when the port is using RGMII. Have the adjust_link function look at
the phy interface type and enable this delay as requested.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>