net: phy: mscc: consolidate a common RGMII delay implementation

It looks like the VSC8584 PHY driver is rolling its own RGMII delay
configuration code, despite the fact that the logic is mostly the same.

In fact only the register layout and position for the RGMII controls has
changed. So we need to adapt and parameterize the PHY-dependent bit
fields when calling the new generic function.

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