dt-bindings: iio/dac: adi,ad5766: Add missing type to 'output-range-microvolts'

'output-range-microvolts' is missing a type definition. '-microvolts' is
not a standard unit (should be '-microvolt'). As the property is already
in use, add a type reference.

Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20220719215152.1877776-1-robh@kernel.org