coresight: disable and enable etmv4 across cpu hotplug

Ensure CoreSight ETMv4 trace unit is disabled before core is
hotplugged off and subsequently enabled when core is online
to prevent race conditions which may arise between cpu hotplug
and enabling or disabling the trace unit.

Change-Id: I51dce88b22f8266d1bb14bc6c36c63c0f9bca0a6
Signed-off-by: Aparna Das <adas@codeaurora.org>