KVM: arm64: nv: Add include containing the VNCR_EL2 offsets

VNCR_EL2 points to a page containing a number of system registers
accessed by a guest hypervisor when ARMv8.4-NV is enabled.

Let's document the offsets in that page, as we are going to use
this layout.

Reviewed-by: Oliver Upton <oliver.upton@linux.dev>
Signed-off-by: Marc Zyngier <maz@kernel.org>