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.comSigned-off-by: Vinod Koul <vkoul@kernel.org>

drivers/phy/microchip/lan966x_serdes.c

@@ -96,6 +96,8 @@ static const struct serdes_mux lan966x_serdes_muxes[] = {
 	SERDES_MUX_SGMII(SERDES6G(1), 3, HSIO_HW_CFG_SD6G_1_CFG,
 			 HSIO_HW_CFG_SD6G_1_CFG_SET(1)),
 
+	SERDES_MUX_SGMII(SERDES6G(2), 4, 0, 0),
+
 	SERDES_MUX_RGMII(RGMII(0), 2, HSIO_HW_CFG_RGMII_0_CFG |
 			 HSIO_HW_CFG_RGMII_ENA |
 			 HSIO_HW_CFG_GMII_ENA,