dt-bindings: iio: adc: adi,ad7606: add conditions

Since the driver supports several parts that present differences in
their layout and behaviour, it is necessary to describe the differences
from one chip to another.

Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Signed-off-by: Guillaume Stols <gstols@baylibre.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>