feat(trbe): enable access to trace buffer control registers from lower NS EL

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

Change-Id: I285a672ccd395eebd377714c992bb21062a729cc
Signed-off-by: Manish V Badarkhe <Manish.Badarkhe@arm.com>
diff --git a/bl31/bl31.mk b/bl31/bl31.mk
index 1fdf545..dc3d3c0 100644
--- a/bl31/bl31.mk
+++ b/bl31/bl31.mk
@@ -90,6 +90,10 @@
 BL31_SOURCES		+=	lib/extensions/mpam/mpam.c
 endif
 
+ifeq (${ENABLE_TRBE_FOR_NS},1)
+BL31_SOURCES		+=	lib/extensions/trbe/trbe.c
+endif
+
 ifeq (${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