docs: fix 'make htmldocs' warning in perf

Fix following 'make htmldocs' warnings:
./Documentation/admin-guide/perf/hisi-pcie-pmu.rst: WARNING:
document isn't included in any toctree

Fixes: c8602008 ("docs: perf: Add description for HiSilicon PCIe PMU driver")
Signed-off-by: Wan Jiabing <wanjiabing@vivo.com>
Reviewed-by: John Garry <john.garry@huawei.com>
Link: https://lore.kernel.org/r/20220228031700.1669086-1-wanjiabing@vivo.comSigned-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/admin-guide/perf/index.rst

@@ -8,6 +8,7 @@ Performance monitor support
    :maxdepth: 1
 
    hisi-pmu
+   hisi-pcie-pmu
    imx-ddr
    qcom_l2_pmu
    qcom_l3_pmu