docs/perf: Add ampere_cspmu to toctree to fix a build warning

docs/perf: Add ampere_cspmu to toctree to fix a build warning

Add ampere_cspmu to toctree in order to address the following warning
produced when building documents:

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

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Closes: https://lore.kernel.org/all/20231011172250.5a6498e5@canb.auug.org.au/
Fixes: 53a810ad3c5c ("perf: arm_cspmu: ampere_cspmu: Add support for Ampere SoC PMU")
Signed-off-by: Ilkka Koskinen <ilkka@os.amperecomputing.com>
Link: https://lore.kernel.org/r/20231012074103.3772114-1-ilkka@os.amperecomputing.com
Signed-off-by: Will Deacon <will@kernel.org>