cxl/test: Standardize CXL auto region size

cxl/test: Standardize CXL auto region size

Create a global define for the size of the mock CXL auto region used
in cxl_test. Remove the declared size in mock_init_hdm_decoder()
function.

Reviewed-by: Jonathan Cameron <jonathan.cameron@huawei.com>
Tested-by: Alison Schofield <alison.schofield@intel.com>
Reviewed-by: Alison Schofield <alison.schofield@intel.com>
Reviewed-by: Fabio M. De Francesco <fabio.m.de.francesco@linux.intel.com>
Link: https://patch.msgid.link/20251117144611.903692-2-dave.jiang@intel.com
Signed-off-by: Dave Jiang <dave.jiang@intel.com>