net: dsa: sja1105: Add RGMII delay support for P/Q/R/S chips

As per the DT phy-mode specification, RGMII delays are applied by the
MAC when there is no PHY present on the link.

Signed-off-by: Vladimir Oltean <olteanv@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>