dt-bindings: iio: adc: Update mcp342x binding for the mcp3421

The mcp3421 is the single channel variant of the mcp342x family and
can be supported by the mcp342x driver.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Cc: devicetree@vger.kernel.org
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>