net/phy: Add VSC8574 support

The VSC8574 is a quad-port Gigabit Ethernet transceiver with four SerDes
interfaces for quad-port dual media capability.

Signed-off-by: Shaohui Xie <Shaohui.Xie@freescale.com>
Signed-off-by: Andy Fleming <afleming@gmail.com>
Signed-off-by: Shruti Kanetkar <Shruti@freescale.com>
Signed-off-by: David S. Miller <davem@davemloft.net>