dt-bindings: iio: adc: mcp320x: Use vendor prefix compatible strings

Update the example to use the compatible string including the
vendor prefix instead of the ones deprecated in 3a872138.

Signed-off-by: Fabian Mewes <architekt@coding4coffee.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>