arm64: dts: ti: k3-am64-main: Add DT node for USB subsystem

Add DT node for the single USB subsystem in main dtsi file.

Signed-off-by: Aswath Govindraju <a-govindraju@ti.com>
Signed-off-by: Nishanth Menon <nm@ti.com>
Reviewed-by: Kishon Vijay Abraham I <kishon@ti.com>
Acked-by: Roger Quadros <rogerq@ti.com>
Link: https://lore.kernel.org/r/20210317043007.18272-2-a-govindraju@ti.com