perf cs-etm: Refactor timestamp variable names

perf cs-etm: Refactor timestamp variable names

Remove ambiguity in variable names relating to timestamps.

A later commit will save the sample kernel timestamp in one of the etm
structs, so name all elements appropriately to avoid confusion.

This is also removes some ambiguity arising from the fact that the
--timestamp argument to perf record refers to sample kernel timestamps,
and the /timestamp/ event modifier refers to CS timestamps, so the term
is overloaded.

Signed-off-by: James Clark <james.clark@arm.com>
Reviewed-by: Mathieu Poirier <mathieu.poirier@linaro.org>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Al Grant <al.grant@arm.com>
Cc: Anshuman Khandual <anshuman.khandual@arm.com>
Cc: Branislav Rankov <branislav.rankov@arm.com>
Cc: Denis Nikitin <denik@chromium.org>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: John Garry <john.garry@huawei.com>
Cc: Leo Yan <leo.yan@linaro.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Mike Leach <mike.leach@linaro.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Suzuki Poulouse <suzuki.poulose@arm.com>
Cc: Will Deacon <will@kernel.org>
Cc: linux-arm-kernel@lists.infradead.org
Cc: coresight@lists.linaro.org
Link: https://lore.kernel.org/r/20210510143248.27423-2-james.clark@arm.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>