phy: lan966x: Add missing serdes mux entry

phy: lan966x: Add missing serdes mux entry

According to the datasheet(Table 3-2: Port configuration) the serdes 2
(SD2) can be configured to run QSGMII or SGMII mode. Already the QSGMII
mode is supported in the serdes_muxes list  but was missing the SGMII mode.
In this mode the serdes is connected to the port 4.
Therefore add this entry in the list.

Signed-off-by: Horatiu Vultur <horatiu.vultur@microchip.com>
Link: https://lore.kernel.org/r/20240108205140.1701770-1-horatiu.vultur@microchip.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>