dt-bindings: iio: ad7192: Add mandatory reference voltage source

Add required reference voltage (VRef) supply regulator.

AD7192 requires three independent voltage sources: DVdd, AVdd and VRef
(on REFINx pin pairs).

Fixes: b581f748 ("staging: iio: adc: ad7192: move out of staging")
Signed-off-by: Fabrizio Lamarque <fl.scratchpad@gmail.com>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Cc: <Stable@vger.kernel.org>
Link: https://lore.kernel.org/r/20230530075311.400686-5-fl.scratchpad@gmail.com


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