feat(sys_reg_trace): enable trace system registers access from lower NS ELs

Introduced a build flag 'ENABLE_SYS_REG_TRACE_FOR_NS' to enable trace
system registers access in NS-EL2, or NS-EL1 (when NS-EL2 is
implemented but unused).

Change-Id: Idc1acede4186e101758cbf7bed5af7b634d7d18d
Signed-off-by: Manish V Badarkhe <Manish.Badarkhe@arm.com>
diff --git a/docs/getting_started/build-options.rst b/docs/getting_started/build-options.rst
index 57b880a..15535d4 100644
--- a/docs/getting_started/build-options.rst
+++ b/docs/getting_started/build-options.rst
@@ -781,6 +781,11 @@
   feature for AArch64. The default is 0 and it is automatically disabled when
   the target architecture is AArch32.
 
+- ``ENABLE_SYS_REG_TRACE_FOR_NS``: Boolean option to enable trace system
+  registers access from NS ELs, NS-EL2 or NS-EL1 (when NS-EL2 is implemented
+  but unused). This feature is available if trace unit such as ETMv4.x, and
+  ETE(extending ETM feature) is implemented. This flag is disabled by default.
+
 GICv3 driver options
 --------------------