Implement SubProcessors core for processor

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

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: Redfish Validator Passed

```
curl -k -X GET https://${bmc}/redfish/v1/Systems/system/Processors/cpu0/SubProcessors/core0
{
  "@odata.id": "/redfish/v1/Systems/system/Processors/cpu0/SubProcessors/core0",
  "@odata.type": "#Processor.v1_18_0.Processor",
  "Id": "core0",
  "Name": "SubProcessor",
  "ProcessorType": "Core"
}
```

Verified that below return a link header
```
- GET /redfish/v1/Systems/system/Processors/cpu0/SubProcessors/core0
- HEAD /redfish/v1/Systems/system/Processors/cpu0/SubProcessors/core0
```

Change-Id: I8cee9909ce20fc0bfdd56fb4fc992163546be180
Signed-off-by: George Liu <liuxiwei@inspur.com>
Signed-off-by: Myung Bae <myungbae@us.ibm.com>