refactor(cpufeat): enable FEAT_PAN for FEAT_STATE_CHECKED

refactor(cpufeat): enable FEAT_PAN for FEAT_STATE_CHECKED

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

Add support for runtime detection (ENABLE_FEAT_PAN=2), by splitting
is_armv8_1_pan_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 PAN 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: I58e5fe8d3c9332820391c7d93a8fb9dba4cf754a
Signed-off-by: Andre Przywara <andre.przywara@arm.com>