arm64: dts: ti: k3-j7200-common-proc-board: Enable SERDES0

Add sub-nodes to SERDES0 DT node to represent SERDES0 is connected
to PCIe and QSGMII (multi-link SERDES).

Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>
Signed-off-by: Nishanth Menon <nm@ti.com>
Reviewed-by: Vignesh Raghavendra <vigneshr@ti.com>
Link: https://lore.kernel.org/r/20210105151421.23237-6-kishon@ti.com