UPSTREAM: coresight: tmc-etr: Create per-thread buffer allocation function

Buffer allocation is different when dealing with per-thread and
CPU-wide sessions.  In preparation to support CPU-wide trace scenarios
simplify things by keeping allocation functions for both type separate.

Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>
Tested-by: Leo Yan <leo.yan@linaro.org>
Tested-by: Robert Walker <robert.walker@arm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

(Upstream commit e553a8aef4c300530844fe096c10975548ea26cb).

Bug: 140266694
Change-Id: I4bb7fb65a98e93181ea61dced9639e18f937511a
Signed-off-by: Yabin Cui <yabinc@google.com>