Add support for cortex-a53 variants when building 64-bit.

Change-Id: I3f1fb5dbde731d9c3d6db26a46bc7f0f54d8e071
diff --git a/core/combo/arch/arm64/armv8-a.mk b/core/combo/arch/arm64/armv8-a.mk
index edc0497..5e27e5a 100644
--- a/core/combo/arch/arm64/armv8-a.mk
+++ b/core/combo/arch/arm64/armv8-a.mk
@@ -1 +1,5 @@
-arch_variant_cflags :=
+ifneq (,$(filter cortex-a53,$(TARGET_$(combo_2nd_arch_prefix)CPU_VARIANT)))
+	arch_variant_cflags := -mcpu=cortex-a53
+else
+	arch_variant_cflags :=
+endif