The ADIN1300 chip supports RGMII, RMII & MII modes. Default (if
unconfigured) is RGMII.
This change adds support for configuring these modes via the device
registers.

For RGMII with internal delays (modes RGMII_ID,RGMII_TXID, RGMII_RXID),
the default delay is 2 ns. This can be configurable and will be done in
a subsequent change.
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Signed-off-by: David S. Miller <davem@davemloft.net>