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>

Documentation/devicetree/bindings/iio/adc/mcp3422.txt

-* Microchip mcp3422/3/4/6/7/8 chip family (ADC)
+* Microchip mcp3421/2/3/4/6/7/8 chip family (ADC)
 
 Required properties:
  - compatible: Should be
+	"microchip,mcp3421" or
 	"microchip,mcp3422" or
 	"microchip,mcp3423" or
 	"microchip,mcp3424" or