iio: Documentation: Add documentation for rotation from north sensor usage attributes

Added documentation for the sysfs attributes supported by the rotation from north
sensor.

Signed-off-by: Reyad Attiyat <reyad.attiyat@gmail.com>
Acked-by: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>