iio: buffer-dmaengine: Add module information

Make sure that the industrialio-buffer-dmaengine has proper license
information so it can be build as a module and loaded without tainting the
kernel.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>