dt-bindings: iio: adc: microchip,mcp3201: fix interface type (I2C -> SPI)

This family of ADCs uses SPI, not I2C.

Signed-off-by: Jan Luebbe <jlu@pengutronix.de>
Link: https://lore.kernel.org/r/20220216162312.4064-1-jlu@pengutronix.de


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>