dt-bindings: iio: adc: ad7380: fix ad7380-4 reference supply

ad7380-4 is the only device from ad738x family that doesn't have an
internal reference. Moreover its external reference is called REFIN in
the datasheet while all other use REFIO as an optional external
reference. If refio-supply is omitted the internal reference is
used.

Fix the binding by adding refin-supply and makes it required for
ad7380-4 only.

Fixes: 1a291cc8 ("dt-bindings: iio: adc: ad7380: add support for ad738x-4 4 channels variants")
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Reviewed-by: David Lechner <dlechner@baylibre.com>
Signed-off-by: Julien Stephan <jstephan@baylibre.com>
Link: https://patch.msgid.link/20241022-ad7380-fix-supplies-v3-1-f0cefe1b7fa6@baylibre.com


Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>