dt-bindings: iio: adc: adi,ad4080: add support for AD4081

dt-bindings: iio: adc: adi,ad4080: add support for AD4081

Add device tree binding support for the AD4081 20-bit SAR ADC.
Add adi,ad4081 to the compatible enum.

A fallback compatible string to adi,ad4080 is not appropriate as the
AD4081 has a different LVDS CNV clock count maximum (2 vs 7), requiring
different driver configuration.

Acked-by: Conor Dooley <conor.dooley@microchip.com>
Signed-off-by: Antoniu Miclaus <antoniu.miclaus@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>