coresight: etm4x: Remove the redundant DSB

As recommended in section 4.3.7 "Synchronization when using the
memory-mapped interface" of ARM IHI0064H.b:

  When using the memory-mapped interface to program the trace unit, the
  trace analyzer must ensure that writes have completed, to ensure that
  the trace unit is fully programmed and either enabled or disabled.

  To ensure writes have completed, the trace analyzer can do ...

  If the memory marked is as Device-nGnRE or stronger, read back the
  value of any register in the trace unit. This relies on peripheral
  coherence order defined in the Arm architecture.

Polling TRCSTATR ensures the previous write has completed. Therefore,
removes the redundant DSB barrier in the enabling flow.

Update the comment in the disable flow for consistency.

Reviewed-by: Yeoreum Yun <yeoreum.yun@arm.com>
Signed-off-by: Leo Yan <leo.yan@arm.com>
Reviewed-by: Mike Leach <mike.leach@linaro.org>
Signed-off-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Link: https://lore.kernel.org/r/20251111-arm_coresight_power_management_fix-v6-7-f55553b6c8b3@arm.com

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