dt-bindings: iio: ad7192: Fix Regulator Properties

The AD7192 binding describes two regulator properties, avdd-supply and
dvdd-supply, but describes it as a constant string that must be avdd and
dvdd. This is wrong since a *-supply property is actually a phandle, and
results in warnings when the example is validated (or any device tree using
that device, for that matter).

Let's remove that requirement.

Fixes: f7356e47 ("dt-bindings: iio: adc: ad7192: Add binding documentation for AD7192")
Signed-off-by: Maxime Ripard <mripard@kernel.org>
Signed-off-by: Rob Herring <robh@kernel.org>