UPSTREAM: coresight: Add helper for inserting synchronization packets

Right now we open code filling the trace buffer with synchronization
packets when the circular buffer wraps around in different drivers.
Move this to a common place. While at it, clean up the barrier_pkt
array to strip off the trailing '\0'.

Cc: Mathieu Poirier <mathieu.poirier@linaro.org>
Cc: Mike Leach <mike.leach@linaro.org>
Signed-off-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

(Upstream commit 6f755e85c332de532d1bbcf1c9769aad08e8cb41).

Bug: 140266694
Change-Id: I290fa5e22a3c10ca083a714448ad9429825a80c6
Signed-off-by: Yabin Cui <yabinc@google.com>