coresight: add dsb sy barrier to stm data writes

Add dsb sy barrier to stm data writes to avoid getting into a
situation where many writes are outstanding due to stm logging.
This helps avoid a cpu errata condition that gets triggerred with
heavy stm logging.

Change-Id: I7664fa3ccfcd8ac05514eb5e4db9441db9d809a2
Signed-off-by: Pratik Patel <pratikp@codeaurora.org>