arm64: KVM: define HYP and Stage-2 translation page flags

Add HYP and S2 page flags, for both normal and device memory.

Reviewed-by: Christopher Covington <cov@codeaurora.org>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>