To allow for assigning a suitable parent to the struct pmu device
update the documentation to describe the device via the event_source
bus where it will remain accessible.

For the ABI documention file also rename the file as it is named
after the path.
Reviewed-by: Yicong Yang <yangyicong@hisilicon.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Signed-off-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Link: https://lore.kernel.org/r/20240412161057.14099-30-Jonathan.Cameron@huawei.com