Implement SubProcessors for processor collection

The SubProcessors is a collection under the processor collection
schema. The association objects, (containing, contained_by), are
used to link the processor.

The association between processor and core have been documented in
phosphor-dbus-interfaces [1]

[1] https://github.com/openbmc/phosphor-dbus-interfaces/commit/8c79b1dc0270d01c0b713a345c8ec39533c542e4

Tested:

- GET cpu and cpu subprocessors
```
curl -k -X GET https://${bmc}/redfish/v1/Systems/system/Processors/cpu0
{
  "@odata.id": "/redfish/v1/Systems/system/Processors/cpu0",
  "@odata.type": "#Processor.v1_18_0.Processor",
  "Id": "cpu0",
 ...
  "SubProcessors": {
    "@odata.id": "/redfish/v1/Systems/system/Processors/cpu0/SubProcessors"
  },
  ...
}
```

```
curl -k -X GET https://${bmc}/redfish/v1/Systems/system/Processors/cpu0/SubProcessors
{
  "@odata.id": "/redfish/v1/Systems/system/Processors/cpu0/SubProcessors",
  "@odata.type": "#ProcessorCollection.ProcessorCollection",
  "Members": [
    {
      "@odata.id": "/redfish/v1/Systems/system/Processors/cpu0/SubProcessors/core0"
    },
    {
      "@odata.id": "/redfish/v1/Systems/system/Processors/cpu0/SubProcessors/core1"
    },
    ...
  ],
  "Members@odata.count": 4,
  "Name": "SubProcessor Collection"
}
```

- Verified that below return a link header
- GET /redfish/v1/Systems/system/Processors/cpu0/SubProcessors
- HEAD /redfish/v1/Systems/system/Processors/cpu0/SubProcessors
- Redfish Validator Passed

Change-Id: If155b97b0c782d82541c00ecf5ee70cb0180f71f
Signed-off-by: George Liu <liuxiwei@ieisystem.com>
Signed-off-by: Nikhil Namjoshi <nikhilnamjoshi@google.com>
Signed-off-by: Myung Bae <myungbae@us.ibm.com>