dt-bindings: iio: ad9467: add entries for for AD9434 & AD9265 ADCs

Add entries for the AD9434 & AD9265 high-speed ADCs which are supported by
the 'ad9467' driver.
Better describe the family of ADCs similar to AD9467 in the description.

Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Link: https://lore.kernel.org/r/20200924080518.96410-5-alexandru.ardelean@analog.com