phy: ocelot-serdes: Add support for SERDES6G muxing

Adding support for SERDES6G muxing required for QSGMII mode of operation.

Signed-off-by: Kavya Sree Kotagiri <kavyasree.kotagiri@microchip.com>
Signed-off-by: Quentin Schulz <quentin.schulz@bootlin.com>
Signed-off-by: Steen Hegelund <Steen.Hegelund@microchip.com>
Co-developed-by: Quentin Schulz <quentin.schulz@bootlin.com>
Co-developed-by: Steen Hegelund <Steen.Hegelund@microchip.com>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>