drm/i915/guc: add dbgfs_node member kernel-doc

drivers/gpu/drm/i915/gt/uc/intel_guc.h:274: warning: Function parameter or member 'dbgfs_node' not described in 'intel_guc'

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/b0f681dd82289dd86da78c6242411e8d812e51a1.1683041799.git.jani.nikula@intel.com