dt-bindings: iio: adc: microchip,mcp3201 yaml conversion.

Drops the deprecated compatibles without the vendor name.
Whilst the driver continues to support these for old dt blobs,
any dt bindings that are actuallly verified against this document should
be fixed to add the vendor name.

Added the #io-channel-cells property to allow for consumers.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Cc: Oskar Andero <oskar.andero@gmail.com>
Link: https://lore.kernel.org/r/20200809111753.156236-2-jic23@kernel.org