scsi/cxlflash: Deprecate driver

We intend to remove the cxlflash driver in an upcoming release. It is
already marked as Obsolete in MAINTAINERS.

The cxlflash driver has received minimal maintenance for some time, and
the CAPI Flash hardware that uses it is no longer commercially available.

Add a warning message on probe and change Kconfig to label the driver as
deprecated and not build the driver by default.

Signed-off-by: Andrew Donnellan <ajd@linux.ibm.com>
Reviewed-by: Frederic Barrat <fbarrat@linux.ibm.com>
Signed-off-by: Madhavan Srinivasan <maddy@linux.ibm.com>
Link: https://patch.msgid.link/20241210054055.144813-3-ajd@linux.ibm.com