net: phy: mscc: add support for RGMII MAC mode

This patch adds support for connecting VSC8584 PHYs to the MAC using
RGMII.

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>