iio: adc: ad4080: add support for AD4880 dual-channel ADC

iio: adc: ad4080: add support for AD4880 dual-channel ADC

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. The driver uses spi_new_ancillary_device() to
create an additional SPI device for the second channel, allowing both
channels to share the same SPI bus with different chip selects.

Reviewed-by: David Lechner <dlechner@baylibre.com>
Reviewed-by: Nuno Sá <nuno.sa@analog.com>
Signed-off-by: Antoniu Miclaus <antoniu.miclaus@analog.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>