net: enetc: allow phy-mode = "1000base-x"

The driver code proper is handled by the lynx_pcs. The enetc just needs
to populate phylink's supported_interfaces array, and return true for
this phy-mode in enetc_port_has_pcs(), such that it creates an internal
MDIO bus through which the Lynx PCS registers are accessed.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Link: https://lore.kernel.org/r/20240103113445.3892971-1-vladimir.oltean@nxp.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>