KVM: arm/arm64: Add kvm_vcpu_load_sysregs and kvm_vcpu_put_sysregs

As we are about to move a bunch of save/restore logic for VHE kernels to
the load and put functions, we need some infrastructure to do this.

Reviewed-by: Andrew Jones <drjones@redhat.com>
Acked-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Christoffer Dall <christoffer.dall@linaro.org>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>