cxl: Add documentation for userspace APIs

This documentation gives an overview of the hardware architecture, userspace
APIs via /dev/cxl/afuM.N and the syfs files. It also adds a MAINTAINERS file
entry for cxl.

Signed-off-by: Ian Munsie <imunsie@au1.ibm.com>
Signed-off-by: Michael Neuling <mikey@neuling.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
diff --git a/Documentation/powerpc/00-INDEX b/Documentation/powerpc/00-INDEX
index a68784d..6fd0e8b 100644
--- a/Documentation/powerpc/00-INDEX
+++ b/Documentation/powerpc/00-INDEX
@@ -11,6 +11,8 @@
 cpu_features.txt
 	- info on how we support a variety of CPUs with minimal compile-time
 	options.
+cxl.txt
+	- Overview of the CXL driver.
 eeh-pci-error-recovery.txt
 	- info on PCI Bus EEH Error Recovery
 firmware-assisted-dump.txt