net: dsa: mv88e6xxx: add port's RGMII delay setter

Some chips such as 88E6352 and 88E6390 can be programmed to add delays
to RXCLK for IND inputs or to GTXCLK for OUTD outputs when port is in
RGMII mode.

Add a port function to program such delays according to the provided PHY
interface mode.

Signed-off-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Signed-off-by: David S. Miller <davem@davemloft.net>