arm64: Detect FEAT_DoubleFault2

KVM will soon support FEAT_DoubleFault2. Add a descriptor for the
corresponding ID register field.

Reviewed-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20250708172532.1699409-3-oliver.upton@linux.dev


Signed-off-by: Oliver Upton <oliver.upton@linux.dev>