dt-bindings: iio: adc: adi,ad4080: add support for AD4082, AD4085 and AD4088

Add device tree binding support for AD4082, AD4085 and AD4088 SAR ADCs.
Add adi,ad4082, adi,ad4085 and adi,ad4088 to the compatible enum.

A fallback compatible string is not appropriate for these devices as they
differ in LVDS CNV clock count maximum from their base variants:
- AD4082 (20-bit) vs AD4080: lvds_cnv_clk_cnt_max 8 vs 7
- AD4085 (16-bit) vs AD4084: lvds_cnv_clk_cnt_max 8 vs 2
- AD4088 (14-bit) vs AD4087: lvds_cnv_clk_cnt_max 8 vs 1

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>