docs: perf: Include hns3-pmu.rst in toctree to fix 'htmldocs' WARNING

mainline inclusion
from mainline-arm64-upstream
commit aaaee7b5
category: feature
bugzilla: https://gitee.com/openeuler/kernel/issues/I5KAX7
CVE: NA

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=aaaee7b55c9e



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After commit 39915b6b ("drivers/perf: hisi: Add description for HNS3
PMU driver"),building the 'htmldocs' target results in the following
warning:

 | Documentation/admin-guide/perf/hns3-pmu.rst: WARNING: document isn't included in any toctree

Add 'hns3-pmu' to the perf toctree to silence the warning.

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Will Deacon <will@kernel.org>
Signed-off-by: Jiantao Xiao <xiaojiantao1@h-partners.com>
Reviewed-by: Xiongfeng Wang <wangxiongfeng2@huawei.com>
Reviewed-by: Yang Jihong <yangjihong1@huawei.com>
Reviewed-by: Jian Shen <shenjian15@huawei.com>
Signed-off-by: Zheng Zengkai <zhengzengkai@huawei.com>