net: phy: Add driver for Motorcomm yt8821 2.5G ethernet phy

Add a driver for the motorcomm yt8821 2.5G ethernet phy. Verified the
driver on BPI-R3(with MediaTek MT7986(Filogic 830) SoC) development board,
which is developed by Guangdong Bipai Technology Co., Ltd..

yt8821 2.5G ethernet phy works in AUTO_BX2500_SGMII or FORCE_BX2500
interface, supports 2.5G/1000M/100M/10M speeds, and wol(magic package).
Signed-off-by: Frank Sae <Frank.Sae@motor-comm.com>
Reviewed-by: Sai Krishna <saikrishnag@marvell.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>