arm64: Detect FEAT_SCTLR2

KVM is about to pick up support for SCTLR2. Add cpucap for later use in
the guest/host context switch hot path.

Reviewed-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20250708172532.1699409-2-oliver.upton@linux.dev


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