net: phy: mscc: RGMII skew delay configuration

This patch adds support for configuring the RGMII skew delays in Rx and
Tx. The Rx and Tx skews are set based on the interface mode. By default
their configuration is set to the default value in hardware (0.2ns);
this means the driver do not rely anymore on the bootloader
configuration.

Then based on the interface mode being used, a 2ns delay is added:
- RGMII_ID adds it for both Rx and Tx.
- RGMII_RXID adds it for Rx.
- RGMII_TXID adds it for Tx.

Signed-off-by: Antoine Tenart <antoine.tenart@bootlin.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>