iio/scmi: Add reading "raw" attribute.

Add IIO_CHAN_INFO_RAW to the mask and implement corresponding
reading "raw" attribute in scmi_iio_read_raw.

Signed-off-by: Andriy Tryshnivskyy <andriy.tryshnivskyy@opensynergy.com>
Acked-by: Jyoti Bhayana <jbhayana@google.com>
Link: https://lore.kernel.org/r/20211024091627.28031-3-andriy.tryshnivskyy@opensynergy.com


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