CXL core is growing, and it's already arguably unmanageable. To support
future growth, move core functionality to a new directory and rename the
file to represent just bus support. Future work will remove non-bus
functionality.

Note that mem.h is renamed to cxlmem.h to avoid a namespace collision
with the global ARCH=um mem.h header.
Reported-by: Nkernel test robot <lkp@intel.com>
Signed-off-by: NBen Widawsky <ben.widawsky@intel.com>
Reviewed-by: NJonathan Cameron <Jonathan.Cameron@huawei.com>
Link: https://lore.kernel.org/r/162792537866.368511.8915631504621088321.stgit@dwillia2-desk3.amr.corp.intel.comSigned-off-by: NDan Williams <dan.j.williams@intel.com>