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

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>