dt-bindings: i2c: Convert i2c-mux bindings to DT schema

As some of the example I2C devices don't have schemas yet, change them to
ones that do.

Cc: Peter Rosin <peda@axentia.se>
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/20210526184839.2937899-5-robh@kernel.org