clk: renesas: mstp: Provide dummy attach/detach_dev callbacks

Provide dummy cpg_mstp_{at,de}tach_dev() PM Domain callbacks if CPG/MSTP
support is not included, so the rcar-sysc driver won't have to care
about this.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>