hw/arm/smmuv3: Update comment documenting "stage" property

hw/arm/smmuv3: Update comment documenting "stage" property

When we added support for nested (stage 1 + stage 2) translation
to the SMMU in commit 58377c363291d we forgot to update the
comment that documents the valid values of the "stage" property.
Add the new "nested" value to it.

Fixes: 58377c363291d ("hw/arm/smmuv3: Support and advertise nesting")
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Eric Auger <eric.auger@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Message-id: 20240816161350.3706332-3-peter.maydell@linaro.org