dt-bindings: iio: adi,ad74115: remove ref from -nanoamp

dtschema v2023.06 comes with support for properties with -nanoamp
suffix, thus bindings should not have a ref for it:

  adi,ad74115.yaml: properties:adi,ext1-burnout-current-nanoamp: '$ref' should not be valid under {'const': '$ref'}

Cc: Cosmin Tanislav <demonsingur@gmail.com>
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Acked-by: Rob Herring <robh@kernel.org>
Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/20230712080512.94964-1-krzysztof.kozlowski@linaro.org


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>