clocksource/drivers/sh_cmt: Split start/stop of clock source and events

clocksource/drivers/sh_cmt: Split start/stop of clock source and events

The CMT do a housekeeping such as dealing with runtime PM and
enable/disable clocks when either a clock source is enabled, or when a
new clock event is registered.

Doing this type of housekeeping for when a clock event is registered is
not always possible as it can happen in contexts where holding spinlocks
is not possible. However doing it when registering a clock source is
possible.

As a first step to address this design break apart the CMT start and
stop functions. The path for clock sources need not change, while the
one for clock events need to be reworked in future work.

There is no indented functional change, just breaking the two use-cases
controlled by a flag into two distinct functions.

Signed-off-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Tested-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/20250910142657.1148696-2-niklas.soderlund+renesas@ragnatech.se