dt-bindings: i2c: mux: i2c-demux-pinctrl: Drop i2c-mux.yaml reference

The I2C de-mux is different than an I2C mux, so i2c-mux.yaml is not
relevant and shouldn't be referenced.

Reviewed-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Link: https://lore.kernel.org/r/20240124190552.1551929-1-robh@kernel.org


Signed-off-by: Rob Herring <robh@kernel.org>