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.hpp b/src/topology.hpp
index 4ea5246..467a5ca 100644
--- a/src/topology.hpp
+++ b/src/topology.hpp
@@ -27,6 +27,7 @@
 
     std::unordered_map<PortType, std::vector<Path>> upstreamPorts;
     std::unordered_map<PortType, std::vector<Path>> downstreamPorts;
+    std::set<Path> powerPaths;
     std::unordered_map<Path, BoardType> boardTypes;
     std::unordered_map<BoardName, Path> boardNames;
 };