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