cxl/test: Enforce PMD alignment for volatile mock regions

cxl/test: Enforce PMD alignment for volatile mock regions

cxl_test allocates synthetic CFMWS HPA windows from a gen_pool with
SZ_256M alignment. On arm64 with CONFIG_ARM64_64K_PAGES=y and
CONFIG_PGTABLE_LEVELS=3, PMD_SIZE is 512M, so every CXL region carved
from a volatile window inherits a non-PMD-aligned start, and
cxl_dax_region_probe() -> alloc_dax_region() fails:

"""
    cxl_dax_region dax_region1: probe with driver cxl_dax_region failed
                                with error -12
"""

Enforce that every volatile mock CFMWS is PMD-aligned in both start and
size

Reviewed-by: Dave Jiang <dave.jiang@intel.com>
Acked-by: Kai-Heng Feng <kaihengf@nvidia.com>
Signed-off-by: Richard Cheng <icheng@nvidia.com>
Reviewed-by: Alison Schofield <alison.schofield@intel.com>
Link: https://patch.msgid.link/20260527090332.30002-1-icheng@nvidia.com
Signed-off-by: Dave Jiang <dave.jiang@intel.com>