dts: bindings: Restrict coresight tmc-etr scatter-gather mode

We are about to add the support for ETR builtin scatter-gather mode
for dealing with large amount of trace buffers. However, on some of
the platforms, using the ETR SG mode can lock up the system due to
the way the ETR is connected to the memory subsystem.

In SG mode, the ETR performs READ from the scatter-gather table to
fetch the next page and regular WRITE of trace data. If the READ
operation doesn't complete(due to the memory subsystem issues,
which we have seen on a couple of platforms) the trace WRITE
cannot proceed leading to issues. So, we by default do not
use the SG mode, unless it is known to be safe on the platform.
We define a DT property for the TMC node to specify whether we
have a proper SG mode.

Cc: Mathieu Poirier <[email protected]>
Cc: Mike Leach <[email protected]>
Cc: Mark Rutland <[email protected]>
Cc: John Horley <[email protected]>
Cc: Robert Walker <[email protected]>
Cc: [email protected]
Cc: [email protected]
Reviewed-by: Rob Herring <[email protected]>
Signed-off-by: Suzuki K Poulose <[email protected]>
Signed-off-by: Mathieu Poirier <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>

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