net: phy: Add MAC-IF driver for Microsemi PHYs.

All the review comments updated and resending for review.

This is MAC interface feature.
Microsemi PHY can support RGMII, RMII or GMII/MII interface between MAC and PHY.
MAC-IF function program the right value based on Device tree configuration.

Tested on Beaglebone Black with VSC 8531 PHY.

Signed-off-by: Raju Lakkaraju <Raju.Lakkaraju@microsemi.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>