perf/arm_cspmu: Assign parents for event_source devices

perf/arm_cspmu: Assign parents for event_source devices

Currently all these devices appear directly under /sys/devices/
Only root busses should appear there, so instead assign the pmu->dev
parents to be the platform device.

Link: https://lore.kernel.org/linux-cxl/ZCLI9A40PJsyqAmq@kroah.com/
Reviewed-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Link: https://lore.kernel.org/r/20240412161057.14099-8-Jonathan.Cameron@huawei.com
Signed-off-by: Will Deacon <will@kernel.org>