staging:iio: Make read / write attributes for channel values optional.

Until now all channels have had read/write attributes. This patch
allows for channels where we can't actually read the value (or for
output devices, write it!)

v2 introduces separate elements for processed and raw thus removing
some special case code from the core.  Thanks to Lars-Peter for an
excellent suggestion!

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>