arm64: dts: ti: k3-j721s2: Convert serdes_ln_ctrl node into reg-mux

This removes a dependency on the parent node being a syscon node.
Convert from mmio-mux to reg-mux adjusting node name and properties
as needed.

Signed-off-by: Andrew Davis <afd@ti.com>
Link: https://lore.kernel.org/r/20240124184722.150615-8-afd@ti.com


Signed-off-by: Vignesh Raghavendra <vigneshr@ti.com>