iio: hid-sensor-attributes: Fix gravity sensor scale value not right issue

Scale value include two parts: unit conversion and exponent conversion.
Add gravity unit convert table to fix gravity sensor scale value not
right issue.

Signed-off-by: Song Hongyan <hongyan.song@intel.com>
Acked-by: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>