perf/arm-cmn: Set PMU device parent

perf/arm-cmn: Set PMU device parent

Now that perf supports giving the PMU device a parent, we can use our
platform device to make the relationship between CMN instances and PMU
IDs trivially discoverable, from either nominal direction:

root@crazy-taxi:~# ls /sys/devices/platform/ARMHC600:00 | grep cmn
arm_cmn_0
root@crazy-taxi:~# realpath /sys/bus/event_source/devices/arm_cmn_0/..
/sys/devices/platform/ARMHC600:00

Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Link: https://lore.kernel.org/r/25d4428df1ddad966c74a3ed60171cd3ca6c8b66.1712682917.git.robin.murphy@arm.com
Signed-off-by: Will Deacon <will@kernel.org>