coresight: etm3x: Set synchronisation frequencty to TRM default

Register ETMSYNCFR holds the number of by that need to be generated before
periodic synchronisation packets are inserted in the trace stream.  By
zeroing out the config structure, the current code effectively disable
periodic synchronization.

This patch simply initialise the recommended value for this register as
specified in the technical reference manual.

Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>