dt-bindings: phy: add bindings for Lynx 28G PHY

Add device tree binding for the Lynx 28G SerDes PHY driver used on
Layerscape based SoCs.

Signed-off-by: Ioana Ciornei <ioana.ciornei@nxp.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Signed-off-by: David S. Miller <davem@davemloft.net>