net: phy: Add driver for Motorcomm yt8521 gigabit ethernet phy

Add a driver for the motorcomm yt8521 gigabit ethernet phy. We have verified
 the driver on StarFive VisionFive development board, which is developed by
 Shanghai StarFive Technology Co., Ltd.. On the board, yt8521 gigabit ethernet
 phy works in utp mode, RGMII interface, supports 1000M/100M/10M speeds, and
 wol(magic package).

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