iio: ad5064: Always use external vref if there is no internal vref

If the device does not have an internal reference, there is no
other choice but to use the external reference. In that case,
it does not make much sense to have to specify it.

This patch ensures that the external reference is used if the
device does not feature an internal reference.

Signed-off-by: Paul Cercueil <paul.cercueil@analog.com>
iio: ad5064: Deduce use of external vref from regulators in dt

Having a 'adi,use-external-reference' property in the device tree
is redundant; if regulators are listed as external references in
the device tree, we can safely assume that the external reference
should be used.

Signed-off-by: Paul Cercueil <paul.cercueil@analog.com>