ARM: shmobile: R-Mobile: Use CPG/MSTP Clock Domain attach/detach helpers

The R-Mobile PM Domain driver manages both power domains and a clock
domain.

The clock domain part is very similar to the CPG/MSTP Clock Domain,
which is used on shmobile SoCs without device power domains, except for
the way how clocks suitable for power management are selected:
  - The former uses the first clock tied to the device through the NULL
    con_id, which is a relic from the legacy pm_clk_notifier-based
    method in drivers/sh/pm_runtime.c,
  - The latter looks for suitable clocks in DT, which is more
    future-proof.

All platforms using this driver are now supported in DT-based ARM
multi-platform builds only, hence switch to using the CPG/MSTP Clock
Domain helpers.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>