MAINTAINERS: Add vfio debugfs interface doc link

After adding the debugfs function to the vfio driver,
a new debugfs-vfio file was added.
Signed-off-by: Longfang Liu <liulongfang@huawei.com>
Link: https://lore.kernel.org/r/20231106072225.28577-4-liulongfang@huawei.comSigned-off-by: Alex Williamson <alex.williamson@redhat.com>

MAINTAINERS

@@ -22837,6 +22837,7 @@ M:	Alex Williamson <alex.williamson@redhat.com>
 L:	kvm@vger.kernel.org
 S:	Maintained
 T:	git https://github.com/awilliam/linux-vfio.git
+F:	Documentation/ABI/testing/debugfs-vfio
 F:	Documentation/ABI/testing/sysfs-devices-vfio-dev
 F:	Documentation/driver-api/vfio.rst
 F:	drivers/vfio/