net: pcs: add pcs-lynx 1000BASE-X support

Add support for 1000BASE-X to pcs-lynx for the LX2160A.

This commit prepares the ground work for allowing 1G fiber connections
to be used with DPAA2 on the SolidRun CEX7 platforms.

Reviewed-by: Ioana Ciornei <ioana.ciornei@nxp.com>
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>