KVM: arm64: Enable Address Authentication at EL2 if available

While initializing EL2, enable Address Authentication if detected
from EL1. We still use the EL1-provided keys though.

Acked-by: Andrew Scull <ascull@google.com>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>