refactor(trbe): enable FEAT_TRBE for FEAT_STATE_CHECKED

refactor(trbe): enable FEAT_TRBE for FEAT_STATE_CHECKED

At the moment we only support FEAT_TRBE to be either unconditionally
compiled in, or to be not supported at all.

Add support for runtime detection (ENABLE_TRBE_FOR_NS=2), by splitting
is_feat_trbe_present() into an ID register reading function and a second
function to report the support status. That function considers both
build time settings and runtime information (if needed), and is used
before we access TRBE related registers.

The FVP platform decided to compile in support unconditionally (=1),
even though FEAT_TRBE is an ARMv9 feature, so 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: Iee7f88ea930119049543a8a4a105389997e7692c
Signed-off-by: Andre Przywara <andre.przywara@arm.com>