iio: ad5064: Explicitly configure whether to use external supply

Previously the driver would revert to internal supply if the
external supply couldn't be found. This had multiple problems:
- it caused silently ignored errors when a regulator was intended
  to be supplied, but was not specified correctly.
- if CONFIG_REGULATOR is disabled, regulator_get() will always
  return a dummy regulator, which caused a device to always use
  the external vref mode, even though there is none.

This patch addresses the issue by adding a platform data structure,
containing a boolean field use_external_ref. If the platform data
structure is present and if that boolean is set, the external vref
is used; otherwise the internal vref is used.

In the case where an external vref is wanted but regulator_get()
fails, the driver no longer reverts to using the internal vref,
but returns an error instead.

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