dt-bindings: i2c: i2c-mux-pinctrl: Convert to json-schema

Convert the pinctrl-based I2C mux bindings from the free-form text
format to json-schema.

Signed-off-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20211217165658.2650677-2-thierry.reding@gmail.com