topology: Add powered_by topology

bmcweb already support the power supply command, refer to
https://gerrit.openbmc.org/c/openbmc/bmcweb/+/57668

Use "PowerPort" to add the powered_by association for upstream port, so
that the PowerSupplyCollection in redfish can get the power supply
information.

Tested:
Add the following config in mobo.json:
```
{
    "Name": "Mobo Upstream Port",
    "Type": "Mobo Upstream Port"
},
```
Add the following config in PSU.json:
```
{
    "ConnectsToType": "Mobo Upstream Port",
    "Name": "PSU $BUS Downstream Port",
    "Type": "DownstreamPort",
    "PowerPort": true
},
```
Result for association:
```
{
	"type" : "as",
	"data" : [
		[
			"/xyz/openbmc_project/inventory/system/board/PSU_1"
		]
	]
}
```
Result in bmcweb:
```
$ curl -s -k -H "X-Auth-Token: $token" http://${bmc}/redfish/v1/Chassis/Mobo | jq .Links.Contains
[
  {
    "@odata.id": "/redfish/v1/Chassis/PSU_1"
  }
]
$ curl -k -H "X-Auth-Token: $token" http://${bmc}/redfish/v1/Chassis/Mobo/PowerSubsystem/PowerSupplies/PSU_1
{
  "@odata.id": "/redfish/v1/Chassis/Mobo/PowerSubsystem/PowerSupplies/PSU_1",
  "@odata.type": "#PowerSupply.v1_5_0.PowerSupply",
  ...
}
```

Run unitest for test_topology is PASSED.

Change-Id: Iad10e61417437a41628cf311cdd7893725a5dcde
Signed-off-by: Jeff Lin <JeffLin2@quantatw.com>
diff --git a/src/topology.cpp b/src/topology.cpp
index f2ee22b..94389b9 100644
--- a/src/topology.cpp
+++ b/src/topology.cpp
@@ -29,6 +29,11 @@
 
         downstreamPorts[connectsTo].emplace_back(path);
         boardTypes[path] = boardType;
+        auto findPoweredBy = exposesItem.find("PowerPort");
+        if (findPoweredBy != exposesItem.end())
+        {
+            powerPaths.insert(path);
+        }
     }
     else if (exposesType.ends_with("Port"))
     {
@@ -65,6 +70,11 @@
                     {
                         result[downstream].emplace_back("contained_by",
                                                         "containing", upstream);
+                        if (powerPaths.find(downstream) != powerPaths.end())
+                        {
+                            result[upstream].emplace_back(
+                                "powered_by", "powering", downstream);
+                        }
                     }
                 }
             }