dt-bindings: adc: ad7173: add support for additional models

Add support for: AD7172-2, AD7175-8, AD7177-2.
AD7172-4 does not feature an internal reference, check for external
 reference presence.

Signed-off-by: Dumitru Ceclan <mitrutzceclan@gmail.com>
Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/20240306110956.13167-2-mitrutzceclan@gmail.com


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>