network/system: Add IPv6 Gateway Property

Right now we can only set an IPv4 gateway. To support dual stack systems
we need a way to have an IPv6 gateway.

Change-Id: Id27f29093a1f605fd460ac22f73abdaeb1f85ff6
Signed-off-by: William A. Kennington III <wak@google.com>
diff --git a/xyz/openbmc_project/Network/SystemConfiguration.interface.yaml b/xyz/openbmc_project/Network/SystemConfiguration.interface.yaml
index a5bdd2f..917a5d6 100644
--- a/xyz/openbmc_project/Network/SystemConfiguration.interface.yaml
+++ b/xyz/openbmc_project/Network/SystemConfiguration.interface.yaml
@@ -8,6 +8,12 @@
     - name: DefaultGateway
       type: string
       description: >
-          default gateway of the system.
+          default IPv4 gateway of the system.
+      errors:
+        - xyz.openbmc_project.Common.Error.InvalidArgument
+    - name: DefaultGateway6
+      type: string
+      description: >
+          default IPv6 gateway of the system.
       errors:
         - xyz.openbmc_project.Common.Error.InvalidArgument