net: mvpp2: 1000baseX support

This patch adds the 1000Base-X PHY mode support in the Marvell PPv2
driver. 1000Base-X is quite close the SGMII and uses nearly the same
code path.

Signed-off-by: Antoine Tenart <antoine.tenart@bootlin.com>
Signed-off-by: David S. Miller <davem@davemloft.net>