dt-bindings: iio: adc: ad7292: Update SPDX identifier

Update SPDX identifier to the preferred dual GPL-2.0 OR BSD-2-Clause
licensing.
Signed-off-by: Marcelo Schmitt <marcelo.schmitt1@gmail.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

Documentation/devicetree/bindings/iio/adc/adi,ad7292.yaml

-# SPDX-License-Identifier: GPL-2.0-only
+# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
 %YAML 1.2
 ---
 $id: http://devicetree.org/schemas/iio/adc/adi,ad7292.yaml#