dt-bindings: i3c: Convert cdns,i3c-master to DT schema

Convert the Cadence I3C master to DT schema format. This fixes a warning
as it is used in the i3c.yaml example.

The "nintendo,nunchuk" is not documented by a schema, so change the
example child device to something which is documented.

Signed-off-by: Rob Herring <robh@kernel.org>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/20220422192224.2594098-1-robh@kernel.org