dts: bindings: coresight: ETM system register access only units

Document the bindings for ETMs with system register accesses.

Link: https://lore.kernel.org/r/20210110224850.1880240-27-suzuki.poulose@arm.com
Cc: devicetree@vger.kernel.org
Cc: Mathieu Poirier <mathieu.poirier@linaro.org>
Cc: Mike Leach <mike.leach@linaro.org>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>
Link: https://lore.kernel.org/r/20210201181351.1475223-29-mathieu.poirier@linaro.orgSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/devicetree/bindings/arm/coresight.txt

@@ -34,9 +34,12 @@ its hardware characteristcs.
 					Program Flow Trace Macrocell:
 			"arm,coresight-etm3x", "arm,primecell";
 
-		- Embedded Trace Macrocell (version 4.x):
+		- Embedded Trace Macrocell (version 4.x), with memory mapped access.
 			"arm,coresight-etm4x", "arm,primecell";
 
+		- Embedded Trace Macrocell (version 4.x), with system register access only.
+			"arm,coresight-etm4x-sysreg";
+
 		- Coresight programmable Replicator :
 			"arm,coresight-dynamic-replicator", "arm,primecell";