arm64: dts: ti: k3-j721e-main: Add system controller node and SERDES lane mux

The system controller node manages the CTRL_MMR0 region.
Add serdes_ln_ctrl node which is used for controlling the SERDES lane mux.

Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>
Signed-off-by: Roger Quadros <rogerq@ti.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Tero Kristo <t-kristo@ti.com>