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/Makefile b/Makefile
index 797b2cf..8e789de 100644
--- a/Makefile
+++ b/Makefile
@@ -1143,7 +1143,6 @@
         COT_DESC_IN_DTB \
         USE_SP804_TIMER \
         PSA_FWU_SUPPORT \
-        ENABLE_SYS_REG_TRACE_FOR_NS \
         ENABLE_MPMM \
         ENABLE_MPMM_FCONF \
         SIMICS_BUILD \
@@ -1182,6 +1181,7 @@
         ENABLE_MPAM_FOR_LOWER_ELS \
         ENABLE_RME \
         ENABLE_SPE_FOR_NS \
+        ENABLE_SYS_REG_TRACE_FOR_NS \
         ENABLE_TRF_FOR_NS \
         FW_ENC_STATUS \
         NR_OF_FW_BANKS \