iio: adc: mcp3422: Add mcp3421 support

The mcp3421 is the single channel variant of the mcp342x family. Support
is straight forward, only the channels array has to be added for this
chip.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>