ndtest: Add regions and mappings to the test buses

The bus config array is used to hold the regions and the respective
mappings. This config based interface enables to change the
dimm/region/namespace layouts easily.

Signed-off-by: Santosh Sivaraj <santosh@fossix.org>
Link: https://lore.kernel.org/r/20201222042240.2983755-6-santosh@fossix.org
Signed-off-by: Dan Williams <dan.j.williams@intel.com>

Author: santoshs