cxl/region: Refactor address translation funcs for testing

cxl/region: Refactor address translation funcs for testing

In preparation for adding a test module that exercises the address
translation calculations, extract the core calculations into stand-
alone functions that operate on base parameters without dependencies
on struct cxl_region.

Perform additional parameter validation to protect against a test
module sending bad parameters. Export the validation function, as
well as the three core translation functions for use by test module
cxl_translate only.

This refactoring enables unit testing of the address translation logic
with controlled inputs, while preserving identical functionality in
the existing code paths.

Reviewed-by: Jonathan Cameron <jonathan.cameron@huawei.com>
Reviewed-by: Dave Jiang <dave.jiang@intel.com>
Signed-off-by: Alison Schofield <alison.schofield@intel.com>
Signed-off-by: Dave Jiang <dave.jiang@intel.com>