remoteproc: mediatek: Unprepare SCP clock during system suspend

Prior to commit d935187cfb27 ("remoteproc: mediatek: Break lock
dependency to prepare_lock"), `scp->clk` was prepared and enabled only
when it needs to communicate with the SCP.  The commit d935187cfb27
moved the prepare operation to remoteproc's prepare(), keeping the clock
prepared as long as the SCP is running.

The power consumption due to the prolonged clock preparation can be
negligible when the system is running, as SCP is designed to be a very
power efficient processor.

However, the clock remains prepared even when the system enters system
suspend.  This prevents the underlying clock controller (and potentially
the parent PLLs) from shutting down, which increases power consumption
and may block the system from entering deep sleep states.

Add suspend and resume callbacks.  Unprepare the clock in suspend() if
it was active and re-prepare it in resume() to ensure the clock is
properly disabled during system suspend, while maintaining the "always
prepared" semantics while the system is active.  The driver doesn't
implement .attach() callback, hence it only checks for RPROC_RUNNING.

Fixes: d935187cfb27 ("remoteproc: mediatek: Break lock dependency to prepare_lock")
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Signed-off-by: Tzung-Bi Shih <tzungbi@kernel.org>
Link: https://lore.kernel.org/r/20260206033034.3031781-1-tzungbi@kernel.org
Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>

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