refactor(cpufeat): enable FEAT_DIT for FEAT_STATE_CHECKED

refactor(cpufeat): enable FEAT_DIT for FEAT_STATE_CHECKED

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

Add support for runtime detection (ENABLE_DIT=2), by splitting
is_armv8_4_dit_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).

We use ENABLE_DIT in two occassions in assembly code, where we just set
the DIT bit in the DIT system register.
Protect those two cases by reading the CPU ID register when ENABLE_DIT
is set to 2.

Change the FVP platform default to the now supported dynamic
option (=2), so the right decision can be made by the code at runtime.

Change-Id: I506d352f18e23c60db8cdf08edb449f60adbe098
Signed-off-by: Andre Przywara <andre.przywara@arm.com>