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
diff --git a/hw/arm/smmuv3.c b/hw/arm/smmuv3.c
index 3971976..4c49b5a 100644
--- a/hw/arm/smmuv3.c
+++ b/hw/arm/smmuv3.c
@@ -1981,6 +1981,7 @@
      * Stages of translation advertised.
      * "1": Stage 1
      * "2": Stage 2
+     * "nested": Both stage 1 and stage 2
      * Defaults to stage 1
      */
     DEFINE_PROP_STRING("stage", SMMUv3State, stage),