dt-bindings: iio: adc: ad7606: Fix wrong maxItems value

This patch set the correct value for oversampling maxItems. In the
original example, appears 3 items for oversampling while the maxItems
is set to 1, this patch fixes those issues.

Fixes: 416f882c ("dt-bindings: iio: adc: Migrate AD7606 documentation to yaml")
Signed-off-by: Beniamin Bia <beniamin.bia@analog.com>
Signed-off-by: Rob Herring <robh@kernel.org>