arm64: sysreg: Add layout for VNCR_EL2

Now that we're about to emulate VNCR_EL2, we need its full layout.
Add it to the sysreg file.

Reviewed-by: Oliver Upton <oliver.upton@linux.dev>
Link: https://lore.kernel.org/r/20250514103501.2225951-2-maz@kernel.org


Signed-off-by: Marc Zyngier <maz@kernel.org>