We do not enable scatter-gather mode in the TMC-ETR by default
to prevent malfunctioning of systems where the ETR may not be
properly connected to the memory subsystem to allow for simultaneous
READ/WRITE transactions when used in SG mode. Instead we whitelist
the platforms where we know that it is safe to use the mode.

All revisions of Juno have a proper ETR connection and hence
white list them.

Cc: Mathieu Poirier <mathieu.poirier@linaro.org>
Cc: Mike Leach <mike.leach@linaro.org>
Cc: Liviu Dudau <liviu.dudau@arm.com>
Cc: Lorenzo Pierlisi <lorenzo.pieralisi@arm.com>
Signed-off-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>