arm64: dts: ti: k3-j721e-main.dtsi: Add USB to SERDES MUX

The USB controllers can be connected to one of the 2 SERDESes
using a MUX. Add a MUX controller node fot that.

Signed-off-by: Roger Quadros <rogerq@ti.com>
Signed-off-by: Sekhar Nori <nsekhar@ti.com>
Signed-off-by: Tero Kristo <t-kristo@ti.com>