net/phy: add VSC8574 support

The VSC8574 is a quad-port Gigabit Ethernet transceiver with four SerDes
interfaces for quad-port dual media capability. This driver supports SGMII
and QSGMII MAC mode. For now SGMII mode is tested.

Signed-off-by: Roy Zang <tie-fei.zang@freescale.com>
Signed-off-by: Shaohui Xie <Shaohui.Xie@freescale.com>
Signed-off-by: Andy Fleming <afleming@freescale.com>