clk: renesas: cpg-mssr: Use always-on governor for Clock Domain

As a pure Clock Domain does not have the concept of powering the domain
itself, the CPG/MSTP driver does not provide power_off() and power_on()
callbacks.
However, the genpd core may still perform a dummy power down, causing
/sys/kernel/debug/pm_genpd/pm_genpd_summary to report the domain's
status being "off-0".

Use the always-on governor to make sure the domain is never powered
down, and always shows up as "on" in pm_genpd_summary.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Ulf Hansson <ulf.hansson@linaro.org>