coresight-tmc: Re-use ETR buffer across use cases

We can run into use after free scenario for the buffer memory
if enable and read operations happen simultaneously. Add mutex
protection in order to prevent such a scenario.

Change-Id: I3106564a46a9cffc0db8808ba03f78d76c925eca
Signed-off-by: Satyajit Desai <sadesai@codeaurora.org>