refactor(cpufeat): enable FEAT_CSV2_2 for FEAT_STATE_CHECKED

refactor(cpufeat): enable FEAT_CSV2_2 for FEAT_STATE_CHECKED

At the moment we only support FEAT_CSV2_2 to be either unconditionally
compiled in, or to be not supported at all.

Add support for runtime detection (ENABLE_FEAT_CSV2_2=2), by splitting
is_armv8_0_feat_csv2_2_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 the SCXTNUM_EL2 system register.
Also move the context saving code from assembly to C, and use the new
is_feat_csv2_2_supported() function to guard its execution.

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: I89c7bc883e6a65727fdbdd36eb3bfbffb2196da7
Signed-off-by: Andre Przywara <andre.przywara@arm.com>