arm64/kvm: Add generic v8 KVM target

This patch adds a generic ARM v8 KVM target cpu type for use
by the new CPUs which eventualy ends up using the common sys_reg
table. For backward compatibility the existing targets have been
preserved. Any new target CPU that can be covered by generic v8
sys_reg tables should make use of the new generic target.

Signed-off-by: Suzuki K. Poulose <suzuki.poulose@arm.com>
Acked-by: Marc Zyngier <Marc.Zyngier@arm.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>