docs: perf: Add description for HiSilicon PCIe PMU driver

PCIe PMU Root Complex Integrated End Point(RCiEP) device is supported on
HiSilicon HIP09 platform. Document it to provide guidance on how to
use it.

Reviewed-by: John Garry <john.garry@huawei.com>
Signed-off-by: Qi Liu <liuqi115@huawei.com>
Reviewed-by: Shaokun Zhang <zhangshaokun@hisilicon.com>
Link: https://lore.kernel.org/r/20211202080633.2919-2-liuqi115@huawei.com


Signed-off-by: Will Deacon <will@kernel.org>