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

Convert the pinctrl-based I2C bus demultiplexer Device Tree binding
documentation to json-schema.

Update the example to match reality.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Signed-off-by: Wolfram Sang <wsa@kernel.org>