cxl/registers: Fix Documentation warning

Commit 0f06157e0135 ("cxl/core: Move register mapping infrastructure")
neglected to add a DOC header for the new drivers/core/regs.c file.

Reported-by: Ben Widawsky <ben.widawsky@intel.com>
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Link: https://lore.kernel.org/r/163072206675.2250120.3527179192933919995.stgit@dwillia2-desk3.amr.corp.intel.com
Signed-off-by: Dan Williams <dan.j.williams@intel.com>