Define the Non-Secure timer frame ID for ARM platforms

On Juno and FVP platforms, the Non-Secure System timer corresponds
to frame 1. However, this is a platform-specific decision and it
shouldn't be hard-coded. Hence, this patch introduces
PLAT_ARM_NSTIMER_FRAME_ID which should be used by all ARM platforms
to specify the correct non-secure timer frame.

Change-Id: I6c3a905d7d89200a2f58c20ce5d1e1d166832bba
diff --git a/include/plat/arm/css/common/css_def.h b/include/plat/arm/css/common/css_def.h
index e5005b9..38ff9dd 100644
--- a/include/plat/arm/css/common/css_def.h
+++ b/include/plat/arm/css/common/css_def.h
@@ -113,4 +113,7 @@
 #define PLAT_ARM_TZC_FILTERS		REG_ATTR_FILTER_BIT_ALL
 #define PLAT_ARM_TZC_BASE		0x2a4a0000
 
+/* System timer related constants */
+#define PLAT_ARM_NSTIMER_FRAME_ID	1
+
 #endif /* __CSS_DEF_H__ */