perf/arm-cmn: Fix CMN S3 DTM offset

CMN S3's DTM offset is different between r0px and r1p0, and it
turns out this was not a error in the earlier documentation, but
does actually exist in the design. Lovely.

Cc: stable@vger.kernel.org
Fixes: 0dc2f496 ("perf/arm-cmn: Support CMN S3")
Signed-off-by: Robin Murphy <robin.murphy@arm.com>