dts: bindings: Document device tree binding for CATU

Document CATU device-tree bindings. CATU augments the TMC-ETR
by providing an improved Scatter Gather mechanism for streaming
trace data to non-contiguous system RAM pages.

Cc: devicetree@vger.kernel.org
Cc: frowand.list@gmail.com
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Mathieu Poirier <mathieu.poirier@linaro.org>
Reviewed-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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>