coresight: change STM ATID to allow concurrent trace sources

Change STM traceid from 3 to 16 so that ETM and STM can be used
concurrently if required. ATIDs 0 to 15 are reserved for ETM - one
for each core.

Change-Id: Ic51cf3b8baa9c22be69c952a20281b238750de5d
Signed-off-by: Pratik Patel <pratikp@codeaurora.org>