coresight-etm4x: Conditionally access register TRCEXTINSELR

The TRCEXTINSELR is only implemented if TRCIDR5.NUMEXTINSEL > 0.
To avoid invalid accesses, introduce a check on numextinsel
(derived from TRCIDR5[11:9]) before reading or writing to this register.

Fixes: f5bd5236 ("coresight: etm4x: Convert all register accesses")
Signed-off-by: Yuanfang Zhang <yuanfang.zhang@oss.qualcomm.com>
Reviewed-by: James Clark <james.clark@linaro.org>
Reviewed-by: Mike Leach <mike.leach@linaro.org>
Signed-off-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Link: https://lore.kernel.org/r/20250812-trcextinselr_issue-v2-1-e6eb121dfcf4@oss.qualcomm.com