arm64: dts: k3-am6: Add mux-controller DT node required for muxing SERDES

Add mux-controller DT node as a child node of scm_conf. This is
required for muxing SERDES between USB, PCIe and ICSS2 SGMII.

Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>
Signed-off-by: Tero Kristo <t-kristo@ti.com>