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/bl31/bl31.mk b/bl31/bl31.mk
index dc3d3c0..ccd54bb 100644
--- a/bl31/bl31.mk
+++ b/bl31/bl31.mk
@@ -94,6 +94,10 @@
 BL31_SOURCES		+=	lib/extensions/trbe/trbe.c
 endif
 
+ifeq (${ENABLE_SYS_REG_TRACE_FOR_NS},1)
+BL31_SOURCES		+=      lib/extensions/sys_reg_trace/aarch64/sys_reg_trace.c
+endif
+
 ifeq (${WORKAROUND_CVE_2017_5715},1)
 BL31_SOURCES		+=	lib/cpus/aarch64/wa_cve_2017_5715_bpiall.S	\
 				lib/cpus/aarch64/wa_cve_2017_5715_mmu.S