staging:iio:isl29028 Add IIO_CHAN_INFO_RAW/PROCESSED entries

Precursor to making value read / write attribute optional.
This one stands along as it merged just before the series
doing all the other drivers.

Signed-off-by: Jonathan Cameron <jic23@kernel.org>
Acked-by: Laxman Dewangan <ldewangan@nvidia.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>