drm/xe/doc: Wire up PCIe Gen5 limitations

Append PCIe Gen5 limitations to xe_firmware document.

Signed-off-by: Raag Jadav <[e-mail]>
Reviewed-by: Rodrigo Vivi <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Rodrigo Vivi <[e-mail]>

Documentation/gpu/xe/xe_firmware.rst

···
 .. kernel-doc:: drivers/gpu/drm/xe/xe_guc_pc.c
    :doc: GuC Power Conservation (PC)
 
+PCIe Gen5 Limitations
+=====================
+
+.. kernel-doc:: drivers/gpu/drm/xe/xe_device_sysfs.c
+   :doc: PCIe Gen5 Limitations
+
 Internal API
 ============