cxl: Fix cross-reference in documentation and add deprecation warning

commit 5731d41 ("cxl: Deprecate driver") labelled the cxl driver as
deprecated and moved the ABI documentation to the obsolete/ subdirectory,
but didn't update cxl.rst, causing a warning once ff7ff6e ("docs:
media: Allow creating cross-references for RC ABI") was merged.

Fix the cross-reference, and also add a deprecation warning.

Fixes: 5731d41 ("cxl: Deprecate driver")
Reported-by: Bagas Sanjaya <bagasdotme@gmail.com>
Signed-off-by: Andrew Donnellan <ajd@linux.ibm.com>
Acked-by: Bagas Sanjaya <bagasdotme@gmail.com>
Signed-off-by: Madhavan Srinivasan <maddy@linux.ibm.com>
Link: https://patch.msgid.link/20250219064807.175107-1-ajd@linux.ibm.com

Author: ajdlinux

Documentation/arch/powerpc/cxl.rst

@@ -18,6 +18,7 @@ Introduction
     both access system memory directly and with the same effective
     addresses.
 
+    **This driver is deprecated and will be removed in a future release.**
 
 Hardware overview
 =================
@@ -453,7 +454,7 @@ Sysfs Class
 
     A cxl sysfs class is added under /sys/class/cxl to facilitate
     enumeration and tuning of the accelerators. Its layout is
-    described in Documentation/ABI/testing/sysfs-class-cxl
+    described in Documentation/ABI/obsolete/sysfs-class-cxl
 
 
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