staging:iio: add a raw and processed elements to info_mask

This will allow us to have drivers where the channel value
may not be read or written but other information is available.
Also adds the ability to have both processed and raw access
to a given channel, though in most cases this doesn't make
sense.

Ultimately will lead to simpler code by allowing us to drop
the special case handling for the value reading cases.

Signed-off-by: Jonathan Cameron <jic23@kernel.org>
Acked-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>