dt-bindings: iio: dac: adi,ad5758: Drop 'contains' from 'adi,dc-dc-mode'

'contains' applies to arrays, but 'adi,dc-dc-mode' is a scalar. So drop
'contains' from the 'if' schema.

Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20221111212846.4104059-1-robh@kernel.org
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>