net: phy: marvell10g: support XFI rate matching mode

When the hardware MACTYPE hardware configuration pins are set to "XFI
with Rate Matching" the PHY interface operate at fixed 10Gbps speed. The
MAC buffer packets in both directions to match various wire speeds.

Read the MAC Type field in the Port Control register, and set the MAC
interface speed accordingly.

Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: David S. Miller <davem@davemloft.net>