drm/xe/configfs: Use tree-like output in documentation

When documenting the directories, use an output similar to the `tree`
command and add VFs and missing attributes.

Reviewed-by: Dnyaneshwar Bhadane <dnyaneshwar.bhadane@intel.com>
Link: https://lore.kernel.org/r/20250821-psmi-v5-9-34ab7550d3d8@intel.com
Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>

drivers/gpu/drm/xe/xe_configfs.c

@@ -39,8 +39,16 @@
  * used to configure it::
  *
  *	/sys/kernel/config/xe/
- *		.. 0000:03:00.0/
- *			... survivability_mode
+ *	├── 0000:00:02.0
+ *	│   └── ...
+ *	├── 0000:00:02.1
+ *	│   └── ...
+ *	:
+ *	└── 0000:03:00.0
+ *	    ├── survivability_mode
+ *	    ├── engines_allowed
+ *	    └── enable_psmi
+ *
  *
  * Configure Attributes
  * ====================