iio: accel: adxl355: Replace custom definitions with generic from units.h

The units.h provides MEGA and TERA. Replace custom ones.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20220202173723.8678-1-andriy.shevchenko@linux.intel.com


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>