Documentation: cxl: remove dangling kernel-doc reference

Sphinx reported kernel-doc failure warning, pointing to non-existent
drivers/cxl/region.h (which doesn't also exist throughout repo history):

WARNING: kernel-doc './scripts/kernel-doc -rst -enable-lineno -sphinx-version 2.4.4 -no-doc-sections ./drivers/cxl/region.h' failed with return code 1

Above cause error message to be displayed on htmldocs output.

Delete the reference.

Fixes: 779dd20c

 ("cxl/region: Add region creation support")
Signed-off-by: Bagas Sanjaya <bagasdotme@gmail.com>
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Link: https://lore.kernel.org/r/20220804075448.98241-4-bagasdotme@gmail.com
Signed-off-by: Dan Williams <dan.j.williams@intel.com>