arm64: dts: renesas: ulcb-kf: Fix pca9548 i2c-mux node names

"make dtbs_check":

    arch/arm64/boot/dts/renesas/r8a77951-ulcb-kf.dtb: i2c-switch@71: $nodename:0: 'i2c-switch@71' does not match '^(i2c-?)?mux'
	    From schema: Documentation/devicetree/bindings/i2c/i2c-mux-pca954x.yaml
    arch/arm64/boot/dts/renesas/r8a77951-ulcb-kf.dtb: i2c-switch@71: Unevaluated properties are not allowed ('#address-cells', '#size-cells', 'i2c@4', 'i2c@7' were unexpected)
	    From schema: Documentation/devicetree/bindings/i2c/i2c-mux-pca954x.yaml
    ...

Fix this by renaming all PCA9548 nodes to "i2c-mux", to match the I2C
bus multiplexer/switch DT bindings and the Generic Names Recommendation
in the Devicetree Specification.

While at it, rename the labels too, for uniformity.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/3c32f500b99e598f458336dc4c05ffa60656324e.1669999298.git.geert+renesas@glider.be