feat(cpufeat): enable FEAT_SME for FEAT_STATE_CHECKED

feat(cpufeat): enable FEAT_SME for FEAT_STATE_CHECKED

Add support for runtime detection (ENABLE_SME_FOR_NS=2), by splitting
feat_sme_supported() 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 do SME specific setup.

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: Ida9ccf737db5be20865b84f42b1f9587be0626ab
Signed-off-by: Jayanth Dodderi Chidanand <jayanthdodderi.chidanand@arm.com>