chore(brcm): include cpu_helpers.S for bl2 build

With errata conversion in place for Cortex_A72 cpu,
build fails with undefined reference to functions defined
in cpu_helpers.S file.
Including the same to handle build issues related to
Stingray platform.

Change-Id: I2bab3e5ca0811d83cd9ee43ac33d557afe6575a8
Signed-off-by: Jayanth Dodderi Chidanand <jayanthdodderi.chidanand@arm.com>
diff --git a/plat/brcm/board/stingray/platform.mk b/plat/brcm/board/stingray/platform.mk
index aa2fe86..67413c6 100644
--- a/plat/brcm/board/stingray/platform.mk
+++ b/plat/brcm/board/stingray/platform.mk
@@ -210,7 +210,8 @@
 
 BL2_SOURCES		+=	plat/${SOC_DIR}/driver/ihost_pll_config.c \
 				plat/${SOC_DIR}/src/bl2_setup.c \
-				plat/${SOC_DIR}/driver/swreg.c
+				plat/${SOC_DIR}/driver/swreg.c \
+				lib/cpus/aarch64/cpu_helpers.S
 
 ifeq (${USE_DDR},yes)
 PLAT_INCLUDES		+=	-Iplat/${SOC_DIR}/driver/ddr/soc/include