refactor(cpufeat): enable FEAT_NV2 for FEAT_STATE_CHECKED

refactor(cpufeat): enable FEAT_NV2 for FEAT_STATE_CHECKED

At the moment we only support for FEAT_NV2 to be either unconditionally
compiled in, or to be not supported at all.

Add support for runtime detection (CTX_INCLUDE_NEVE_REGS=2), by
splitting get_armv8_4_feat_nv_support() 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 VNCR_EL2 system register.
Also move the context saving code from assembly to C, and use the new
is_feat_nv2_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: I85b080641995fb72cfd4ac933f7a3f75770c2cb9
Signed-off-by: Andre Przywara <andre.przywara@arm.com>