iio: adc: ad7476: Add IIO_CHAN_INFO_RAW for AD7091R

When CONVST signal is generated internally, IIO_CHAN_INFO_RAW can be
made available for AD7091R for single reads. This patch enables it and
makes supporting more devices by this driver easier.

Signed-off-by: Dragos Bogdan <dragos.bogdan@analog.com>
Signed-off-by: Beniamin Bia <beniamin.bia@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>