iio: adc: ad9467: add support for AD9434 high-speed ADC

The AD9434 is a 12-bit monolithic sampling analog-to-digital converter
(ADC) optimized for high performance, low power, and ease of use. The part
operates at up to a 500 MSPS conversion rate and is optimized for
outstanding dynamic performance in wideband carrier and broadband systems.

All necessary functions, including a sample-and-hold and voltage reference,
are included on the chip to provide a complete signal conversion solution.
The VREF pin can be used to monitor the internal reference or provide an
external voltage reference (external reference mode must be enabled through
the SPI port).

The ADC requires a 1.8 V analog voltage supply and a differential clock
for full performance operation. The digital outputs are LVDS (ANSI-644)
compatible and support twos complement, offset binary format, or Gray code.
A data clock output is available for proper output data timing.

Link: https://www.analog.com/media/en/technical-documentation/data-sheets/AD9434.pdf

The driver supports the same register set as the AD9467, so the support for
this chip is added to the 'ad9467' driver.

Signed-off-by: Michael Hennerich <michael.hennerich@analog.com>
Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Link: https://lore.kernel.org/r/20200924080518.96410-3-alexandru.ardelean@analog.com