docs(n1sdp): add N1SDP PSCI instrumentation data

Change-Id: Id22715cb1d36edf6cb8719f3a0415993f067e7c9
Signed-off-by: Harrison Mutai <harrison.mutai@arm.com>
diff --git a/docs/perf/index.rst b/docs/perf/index.rst
index b83c6e3..0938a17 100644
--- a/docs/perf/index.rst
+++ b/docs/perf/index.rst
@@ -7,6 +7,7 @@
 
    psci-performance-instr
    psci-performance-juno
+   psci-performance-n1sdp
    psci-performance-methodology
    tsp
    performance-monitoring-unit