net: dpaa2-mac: add 1000BASE-X support

Now that pcs-lynx supports 1000BASE-X, add support for this interface
mode to dpaa2-mac. pcs-lynx can be switched at runtime between SGMII
and 1000BASE-X mode, so allow dpaa2-mac to switch between these as
well.

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>