coresight: add spinlock across stm channel allocation

There appears to be a race condition during stm channel allocation where
two processes running in parallel get same channel. Fix this by adding a
spinlock across channel allocation logic until we figure out a root cause
of this issue.

Change-Id: Ifcc655957adae3c58d8b329d8cd5c0551a26d5c6
Signed-off-by: Sarangdhar Joshi <spjoshi@codeaurora.org>