schemas: topology: Add 'BackplanePort' to 'Type'

`test/test_topology.cpp` uses Type `BackplanePort` and the current
topology implementation does not hardcode the identifier for port
associations to be `MB Upstream Port` so it is assumed there was some
intent to allow this value as well.

At least the schema and tests should match and there needs to be some
way to identify a port. Rather than deleting the respective tests, add
this value to the schema.

Tested: Topology Unit Tests Pass

Change-Id: Iee7576e6187269348acdbbbfd4409f4b6584f20b
Signed-off-by: Alexander Hansen <alexander.hansen@9elements.com>
diff --git a/schemas/topology.json b/schemas/topology.json
index b5e1a18..5b9a968 100644
--- a/schemas/topology.json
+++ b/schemas/topology.json
@@ -11,7 +11,7 @@
                 "Type": {
                     "description": "This enum may be extended as described in the design",
                     "type": "string",
-                    "enum": ["MB Upstream Port"]
+                    "enum": ["MB Upstream Port", "BackplanePort"]
                 }
             },
             "required": ["Name", "Type"],
@@ -27,7 +27,7 @@
                 "ConnectsToType": {
                     "description": "Should be equal to the 'Type' property on the upstream port",
                     "type": "string",
-                    "const": "MB Upstream Port"
+                    "enum": ["MB Upstream Port", "BackplanePort"]
                 },
                 "PowerPort": {
                     "type": "boolean"