Update docs with PMU security information

This patch adds information on the PMU configuration registers
and security considerations related to the PMU.

Signed-off-by: Petre-Ionut Tudor <petre-ionut.tudor@arm.com>
Change-Id: I36b15060b9830a77d3f47f293c0a6dafa3c581fb
diff --git a/docs/perf/index.rst b/docs/perf/index.rst
index 0f49b48..1482b80 100644
--- a/docs/perf/index.rst
+++ b/docs/perf/index.rst
@@ -8,7 +8,8 @@
 
    psci-performance-juno
    tsp
+   performance-monitoring-unit
 
 --------------
 
-*Copyright (c) 2019, Arm Limited. All rights reserved.*
+*Copyright (c) 2019-2020, Arm Limited. All rights reserved.*