drivers: adxl362: fix acceleration sample conversion

This driver uses magic numbers which are incorrect. A correct conversion
is done using the LSB/g scale factors given by Table 1 in the
specifications section of the ADXL362 (Rev. F) datasheet. The entire
conversion has also been consolidated into a signal function.

The sensitivity and offset of the accelerometer values actually vary
with the supply voltage and temperature. Using datasheet provided values
for 25 C and 2.0 V is the best we can do. Users will need to apply
sensitivity and offset corrections for their application.

Signed-off-by: Brett Witherspoon <spoonb@cdspooner.com>