dt-bindings: mux: Convert mux controller bindings to schema

Convert the mux controller bindings to DT schema.

Cc: Peter Rosin <peda@axentia.se>
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Acked-by: Wolfram Sang <wsa@kernel.org>
Acked-by: Peter Rosin <peda@axentia.se>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20210526184839.2937899-4-robh@kernel.org