drivers/perf: hisi: Add description for HNS3 PMU driver

HNS3 PMU End Point device is supported on HiSilicon HIP09 platform, so
add document hns3-pmu.rst to provide guidance on how to use it.

Signed-off-by: Guangbin Huang <huangguangbin2@huawei.com>
Reviewed-by: John Garry <john.garry@huawei.com>
Reviewed-by: Shaokun Zhang <zhangshaokun@hisilicon.com>
Link: https://lore.kernel.org/r/20220628063419.38514-2-huangguangbin2@huawei.com


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