coresight: Prepare to allow setting the timestamp interval

coresight: Prepare to allow setting the timestamp interval

Timestamps are currently emitted at the maximum rate possible, which is
much too frequent for most use cases. In the next commit, the timestamp
field will be widened to take a value, so set the interval using the
value now. Granular control is not required, so save space in the config
by interpreting it as 2 ^ timestamp. And then 4 bits (0 - 15) will be
enough to set the interval to be larger than the existing SYNC timestamp
interval.

No sysfs mode support is needed for this attribute because counter
generated timestamps are only configured for Perf mode.

Reviewed-by: Leo Yan <leo.yan@arm.com>
Tested-by: Leo Yan <leo.yan@arm.com>
Signed-off-by: James Clark <james.clark@linaro.org>
Signed-off-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Link: https://lore.kernel.org/r/20251128-james-cs-syncfreq-v8-11-4d319764cc58@linaro.org