refactor(cpufeat): enable SYS_REG_TRACE for FEAT_STATE_CHECKED

At the moment we only support access to the trace unit by system
registers (SYS_REG_TRACE) to be either unconditionally compiled in, or
to be not supported at all.

Add support for runtime detection (ENABLE_SYS_REG_TRACE_FOR_NS=2), by
adding is_feat_sys_reg_trace_supported(). That function considers both
build time settings and runtime information (if needed), and is used
before we access SYS_REG_TRACE related registers.

The FVP platform decided to compile in support unconditionally (=1),
even though this is an optional feature, so it is not available with the
FVP model's default command line.
Change that to the now supported dynamic option (=2), so the right
decision can be made by the code at runtime.

Change-Id: I450a574a4f6bd9fc269887037049c94c906f54b2
Signed-off-by: Andre Przywara <andre.przywara@arm.com>
diff --git a/include/lib/extensions/sys_reg_trace.h b/include/lib/extensions/sys_reg_trace.h
index 74470fe..5915c55 100644
--- a/include/lib/extensions/sys_reg_trace.h
+++ b/include/lib/extensions/sys_reg_trace.h
@@ -9,10 +9,24 @@
 
 #include <context.h>
 
+#if ENABLE_SYS_REG_TRACE_FOR_NS
 #if __aarch64__
 void sys_reg_trace_enable(cpu_context_t *context);
 #else
 void sys_reg_trace_enable(void);
 #endif /* __aarch64__ */
 
+#else /* !ENABLE_SYS_REG_TRACE_FOR_NS */
+
+#if __aarch64__
+static inline void sys_reg_trace_enable(cpu_context_t *context)
+{
+}
+#else
+static inline void sys_reg_trace_enable(void)
+{
+}
+#endif /* __aarch64__ */
+#endif /* ENABLE_SYS_REG_TRACE_FOR_NS */
+
 #endif /* SYS_REG_TRACE_H */