powerpc/dts/fsl: Fix pca954x i2c-mux node names

"make dtbs_check":

    arch/powerpc/boot/dts/fsl/t1040rdb-rev-a.dtb: pca9546@77: $nodename:0: 'pca9546@77' does not match '^(i2c-?)?mux'
           From schema: Documentation/devicetree/bindings/i2c/i2c-mux-pca954x.yaml
    arch/powerpc/boot/dts/fsl/t1024qds.dtb: pca9547@77: Unevaluated properties are not allowed ('#address-cells', '#size-cells', 'i2c@0', 'i2c@2', 'i2c@3' were unexpected)
           From schema: Documentation/devicetree/bindings/i2c/i2c-mux-pca954x.yaml
    ...

Fix this by renaming pca954x nodes to "i2c-mux", to match the I2C bus
multiplexer/switch DT bindings and the Generic Names Recommendation in
the Devicetree Specification.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/6c5d86c49ac170e9d56ab121ea0602f3873849ca.1669999298.git.geert+renesas@glider.be