iio: ad7476: Add support for the ad7091r

Add support for the ad7091r 12 bit ADC to the ad7476 driver. Although the
ad7091r is not really related to any of the other devices supported by this
driver, luckily for us there are not so many ways (which are not totally insane)
how sampling a single channel ADC via SPI can be implemented and support for the
ad7091r can be added to the driver with just a few adjustments.

The ad7091r requires an external "conversion start" pulse to start a sample
conversion. After the conversion has finished the result can be read via SPI. We
depend on a IIO trigger to generate this signal, as a result only sampling in
buffered mode and not in manual mode is available.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>