dt-bindings: iio: adc: adi,ad4030: Add PWM

dt-bindings: iio: adc: adi,ad4030: Add PWM

In setups designed for high speed data rate capture, a PWM is used to
generate the CNV signal that issues data captures from the ADC. Document
the use of a PWM for AD4030 and similar devices.

Reviewed-by: David Lechner <dlechner@baylibre.com>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Signed-off-by: Marcelo Schmitt <marcelo.schmitt@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>