net: phy: Add driver for Motorcomm yt8531 gigabit ethernet phy

Add a driver for the motorcomm yt8531 gigabit ethernet phy. We have
 verified the driver on AM335x platform with yt8531 board. On the
 board, yt8531 gigabit ethernet phy works in utp mode, RGMII
 interface, supports 1000M/100M/10M speeds, and wol(magic package).

Signed-off-by: Frank Sae <Frank.Sae@motor-comm.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>