cxl/core: Add cxl-bus driver infrastructure

Enable devices on the 'cxl' bus to be attached to drivers. The initial
user of this functionality is a driver for an 'nvdimm-bridge' device
that anchors a libnvdimm hierarchy attached to CXL persistent memory
resources. Other device types that will leverage this include:

cxl_port: map and use component register functionality (HDM Decoders)

cxl_nvdimm: translate CXL memory expander endpoints to libnvdimm
	    'nvdimm' objects

cxl_region: translate CXL interleave sets to libnvdimm 'region' objects

The pairing of devices to drivers is handled through the cxl_device_id()
matching to cxl_driver.id values. A cxl_device_id() of '0' indicates no
driver support.

In addition to ->match(), ->probe(), and ->remove() support for the
'cxl' bus introduce MODULE_ALIAS_CXL() to autoload modules containing
cxl-drivers. Drivers are added in follow-on changes.

Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Link: https://lore.kernel.org/r/162379909190.2993820.6134168109678004186.stgit@dwillia2-desk3.amr.corp.intel.com


Signed-off-by: Dan Williams <dan.j.williams@intel.com>