cxl: Deprecate driver

The cxl driver is no longer actively maintained and we intend to remove it
in a future kernel release.

cxl has received minimal maintenance for several years, and is not
supported on the Power10 processor. We aren't aware of any users who are
likely to be using recent kernels.

Change its MAINTAINERS status to obsolete, update the sysfs ABI
documentation accordingly, add a warning message on device probe, change
the Kconfig options to label it as deprecated, and don't build it by
default.

Signed-off-by: Andrew Donnellan <ajd@linux.ibm.com>
Acked-by: Frederic Barrat <fbarrat@linux.ibm.com>
Signed-off-by: Madhavan Srinivasan <maddy@linux.ibm.com>
Link: https://patch.msgid.link/20241210054055.144813-2-ajd@linux.ibm.com