coresight: document binding acronyms

It can be hard for people not familiar with the CoreSight IP blocks
to make sense of the acronyms found in the current bindings.  As such
this patch expands each acronym in the hope of providing a better
description of the IP block they represent.

Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>
Acked-by: Sudeep Holla <sudeep.holla@arm.com>
Reviewed-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Signed-off-by: Rob Herring <robh@kernel.org>