iio: adc: ad4080: add support for AD4086

iio: adc: ad4080: add support for AD4086

Add support for AD4086 14-bit SAR ADC. The AD4086 differs from
AD4080 in resolution (14-bit vs 20-bit) and LVDS CNV clock count
maximum (4 vs 7).

Changes:
- Add AD4086_CHIP_ID definition (0x0056)
- Create ad4086_channel with 14-bit resolution and 16-bit storage
- Add ad4086_chip_info with lvds_cnv_clk_cnt_max = 4
- Register AD4086 in device ID and OF match tables

Signed-off-by: Antoniu Miclaus <antoniu.miclaus@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>