dt-bindings: i2c: i2c-mux-pca954x: Convert to DT schema

Convert the i2c-mux-pca954x binding to DT schema format.

Add the missing compatible for 'nxp,pca9645' which is already in use.

Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.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/20210526184839.2937899-6-robh@kernel.org