UPSTREAM: coresight: tmc-etr: Introduce the notion of reference counting to ETR devices

This patch adds reference counting to struct etr_buf so that, in CPU-wide
trace scenarios, shared buffers can be disposed of when no longer used.

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 57549999b9a0d0c72900d2413ef1e3168bd2e817).

Bug: 140266694
Change-Id: I0a9434fc147b5b66dba7e6e706781db4fb95e963
Signed-off-by: Yabin Cui <yabinc@google.com>