riscv: dts: sophgo: fix mdio node name for CV180X

riscv: dts: sophgo: fix mdio node name for CV180X

As the mdio multipledxer is marked as mdio device, the check complains
the mdio bus number exceed the maximum.

Change the node name to mdio-mux to remove the following warnings:
mdio@3009800 (mdio-mux-mmioreg): mdio@80:reg:0:0: 128 is greater than
the maximum of 31

Fixes: b7945143bc33 ("riscv: dts: sophgo: Add mdio multiplexer device for cv18xx")
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202507140738.XRjv3G8i-lkp@intel.com/
Closes: https://lore.kernel.org/oe-kbuild-all/202507121830.POx2KDVi-lkp@intel.com/
Reviewed-by: Alexander Sverdlin <alexander.sverdlin@gmail.com>
Link: https://lore.kernel.org/r/20250715221349.11034-1-inochiama@gmail.com
Signed-off-by: Inochi Amaoto <inochiama@gmail.com>
Signed-off-by: Chen Wang <unicorn_wang@outlook.com>
Signed-off-by: Chen Wang <wangchen20@iscas.ac.cn>