coresight: tmc: mark symbols static where possible

coresight: tmc: mark symbols static where possible

We get a few warnings when building kernel with W=1:
drivers/hwtracing/coresight/coresight-tmc-etr.c:23:6: warning: no previous prototype for 'tmc_etr_enable_hw' [-Wmissing-prototypes]
drivers/hwtracing/coresight/coresight-tmc-etf.c:25:6: warning: no previous prototype for 'tmc_etb_enable_hw' [-Wmissing-prototypes]
drivers/hwtracing/coresight/coresight-tmc.c:250:9: warning: no previous prototype for ‘trigger_cntr_show’ [-Wmissing-prototypes]
...

In fact, these functions are only used in the file in which they are
declared and don't need a declaration, but can be made static.
so this patch marks these functions with 'static'.

Signed-off-by: Baoyou Xie <baoyou.xie@linaro.org>
Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>