dt-bindings: iio: adc: ad4080: add AD4880 support

dt-bindings: iio: adc: ad4080: add AD4880 support

Add support for the AD4880, a dual-channel 20-bit 40MSPS SAR ADC
with integrated fully differential amplifiers (FDA).

The AD4880 has two independent ADC channels, each with its own SPI
configuration interface. This requires:
- Two entries in reg property for primary and secondary channel
  chip selects
- Two io-backends entries for the two data channels

Acked-by: Conor Dooley <conor.dooley@microchip.com>
Reviewed-by: David Lechner <dlechner@baylibre.com>
Signed-off-by: Antoniu Miclaus <antoniu.miclaus@analog.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>