iio: accel: bma400: Use macros for generic event configuration values

iio: accel: bma400: Use macros for generic event configuration values

Add macros and enums for configuration values used in generic event
handling for activity and inactivity detection. Replace hard-coded
values in activity_event_en() with the new definitions to make the
configuration explicit.

No functional changes are intended.

Signed-off-by: Akshay Jindal <akshayaj.lkd@gmail.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>