KVM: arm64: nv: Add EL2_REG_VNCR()/EL2_REG_REDIR() sysreg helpers

Add two helpers to deal with EL2 registers are are either redirected
to the VNCR page, or that are redirected to their EL1 counterpart.

In either cases, no trap is expected.

THe relevant register descriptors are repainted accordingly.

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