Add Location information for PCIeDevice

This commit is to add location information according to the Redfish
PCIeDevice schema. If the
`xyz.openbmc_project.Inventory.Decorator.LocationCode` interface does
not exist, the location information property is not displayed.
The Redfish PCIeDevice schema includes a Location property under Slot.
Get the Location from PCIeSlot.

ref: https://redfish.dmtf.org/schemas/v1/PCIeDevice.v1_19_0.json

Tested: Validator passed

curl -k https://$bmc/redfish/v1/Systems/system/PCIeDevices/pcie_card2
```
    {
  "@odata.id": "/redfish/v1/Systems/system/PCIeDevices/pcie_card2",
  "@odata.type": "#PCIeDevice.v1_19_0.PCIeDevice",
  "Id": "pcie_card2",
  "Links": {
    "Processors": [
      {
        "@odata.id": "/redfish/v1/Systems/system/Processors/cpu1"
      }
    ],
    "Processors@odata.count": 1
  },
  "Name": "PCIe Device",
  "PCIeFunctions": {
    "@odata.id": "/redfish/v1/Systems/system/PCIeDevices/pcie_card2/PCIeFunctions"
  },
  "PCIeInterface": {
    "LanesInUse": null
  },
  "Slot": {
    "Location": {
      "PartLocation": {
        "ServiceLabel": "U78DA.N00.1234567-P0-C2"
      }
    },
    "SlotType": "FullLength"
  },
  "Status": {
    "Health": "OK",
    "State": "Absent"
  }
}
```

Change-Id: Iba1b98734fdfee43c2995f6c9f1f2660e196814a
Signed-off-by: Lakshmi Yadlapati <lakshmiy@us.ibm.com>
Signed-off-by: Crystal Huang <crzhuang1@gmail.com>