Documentation: trace: Add documentation for TRBE

Add documentation for the TRBE under trace/coresight.

Cc: Jonathan Corbet <[e-mail]>
Cc: [e-mail]
Signed-off-by: Anshuman Khandual <[e-mail]>
[ Split from the TRBE driver patch ]
Signed-off-by: Suzuki K Poulose <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Mathieu Poirier <[e-mail]>

Documentation/trace/coresight/coresight-trbe.rst

···
+.. SPDX-License-Identifier: GPL-2.0
+
+==============================
+Trace Buffer Extension (TRBE).
+==============================
+
+    :Author:   Anshuman Khandual <[e-mail]>
+    :Date:     November 2020
+
+Hardware Description
+--------------------
+
+Trace Buffer Extension (TRBE) is a percpu hardware which captures in system
+memory, CPU traces generated from a corresponding percpu tracing unit. This
+gets plugged in as a coresight sink device because the corresponding trace
+generators (ETE), are plugged in as source device.
+
+The TRBE is not compliant to CoreSight architecture specifications, but is
+driven via the CoreSight driver framework to support the ETE (which is
+CoreSight compliant) integration.
+
+Sysfs files and directories
+---------------------------
+
+The TRBE devices appear on the existing coresight bus alongside the other
+coresight devices::
+
+	>$ ls /sys/bus/coresight/devices
+	trbe0  trbe1  trbe2 trbe3
+
+The ``trbe<N>`` named TRBEs are associated with a CPU.::
+
+	>$ ls /sys/bus/coresight/devices/trbe0/
+        align flag
+
+*Key file items are:-*
+   * ``align``: TRBE write pointer alignment
+   * ``flag``: TRBE updates memory with access and dirty flags