iio: adc: ad4080: add support for AD4084

iio: adc: ad4080: add support for AD4084

Add support for AD4084 16-bit SAR ADC. The AD4084 differs from
AD4080 in resolution (16-bit vs 20-bit) and LVDS CNV clock count
maximum (2 vs 7).

Changes:
- Add AD4084_CHIP_ID definition (0x0054)
- Create ad4084_channel with 16-bit resolution and storage
- Add ad4084_chip_info with appropriate configuration
- Register AD4084 in device ID and OF match tables

Reviewed-by: Nuno Sá <nuno.sa@analog.com>
Signed-off-by: Antoniu Miclaus <antoniu.miclaus@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>