dt-bindings: iio: adc: adi,ad7380: add spi-rx-bus-width property

dt-bindings: iio: adc: adi,ad7380: add spi-rx-bus-width property

Add spi-rx-bus-width property to describe how many SDO lines are wired
up on the ADC. These chips are simultaneous sampling ADCs and have one
SDO line per channel, either 2 or 4 total depending on the part number.

Reviewed-by: Rob Herring (Arm) <robh@kernel.org>
Signed-off-by: David Lechner <dlechner@baylibre.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>