coresight: etm: Add check of trace unit power property

Add check of trace unit power property to enable/disable that
keep the trace unit powered.

Change-Id: I66dbfee16df63480c9309092e61ee39b8b0cd6e8
Signed-off-by: Mao Jinlong <jinlmao@codeaurora.org>
Signed-off-by: Tingwei Zhang <tingwei@codeaurora.org>