UPSTREAM: coresight: tmc-etr buf: Add TMC scatter gather mode backend

Add the support for Scatter-Gather mode to the etr-buf layer.
Since we now have two different modes, we choose the backend
based on a set of conditions, documented in the code.

Cc: Mathieu Poirier <mathieu.poirier@linaro.org>
Signed-off-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

(Upstream commit e8e3b771395d9b75ba5da409b7d20435ddb05a21).

Bug: 140266694
Change-Id: I8bc47e6f7ec1807577519e574176f7666a83c33a
Signed-off-by: Yabin Cui <yabinc@google.com>