staging:iio:imu Add IIO_CHAN_INFO_RAW entries to all drivers.

Precursor to making value read / write attribute optional.
No processed values in IMU.

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>