coresight: graceful etm enable/disable for offline cores

Fail ETM enable if the core is offline so that it is obvious to
the user that the operation failed. ETM disable cannot fail so
disable ETM during hotplugg off if it is user enabled and
re-enable ETM during hotplugg on if it is user enabled.

Change-Id: I24120582ff6d3c5f3598baff962dc2ef910c3a1e
Signed-off-by: Pratik Patel <pratikp@codeaurora.org>