perf/arm-cmn: Allow counting non-idle cycles

Although since commit c5b15ddf ("perf/arm-cmn: Make cycle counts
less surprising") the dtc_cycles event now has the generally more useful
behaviour expected for bandwidth and frequency calculations, there are
still some potential niche uses for observing CMN's notion of "idle" via
DTC clock gating, at least on configurations with only one DTC domain.
Luckily things are now the right way round for the standard exclude_idle
attribute to seem a good enough fit, so we can wire that up.

This then allows fun tricks like getting a rough view of the level of
interconnect activity:

$ perf stat -r10 -a -earm_cmn_0/dtc_cycles/{,I} sleep 1

 Performance counter stats for 'system wide' (10 runs):

        1607593396      arm_cmn_0/dtc_cycles/        ( +-  0.01% )  (50.15%)
           9030986      arm_cmn_0/dtc_cycles/I       ( +- 15.08% )  (49.85%)

          1.005074 +- 0.000135 seconds time elapsed  ( +-  0.01% )

...seems like there's not much background traffic on this otherwise-idle
machine, and CMN-600's clock gating is doing a pretty good job!

Signed-off-by: Robin Murphy <robin.murphy@arm.com>