staging:iio:light Add IIO_CHAN_INFO_RAW/PROCESSED entries to all drivers.

Precursor to making value read / write attribute optional.
Note that minimal change route taken here. The read_raw callbacks
in both drivers could do fewer checks to identify the channel than
they now do.

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>