iio: accel: bma400: Use index-based register addressing and lookup

iio: accel: bma400: Use index-based register addressing and lookup

Introduce formula-based macros to compute GEN INTR configuration register
addresses from the interrupt number and register index. This reduces the
need for 22 explicit register macros to three base definitions.

Add a centralized lookup table keyed by IIO event direction and replace
get_gen_config_reg() with a helper integrated with this table.

Apply these changes across the affected callbacks to ensure consistent
access to generic interrupt registers.

No functional changes are intended.

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