cxl/core: Move register mapping infrastructure

The register mapping infrastructure is large enough to move to its own
compilation unit. This also cleans up an unnecessary include of <mem.h>
core/bus.c.

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Ben Widawsky <ben.widawsky@intel.com>
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Link: https://lore.kernel.org/r/162800068975.665205.12895551621746585289.stgit@dwillia2-desk3.amr.corp.intel.com
Signed-off-by: Dan Williams <dan.j.williams@intel.com>