Refactor SystemPCIeFunction method

Move SystemPCIeFunction to a separate method, and refactor the code.
Validate the PCIe device path and add link header.

Tested: Validator passed

```
{
  "@odata.id": "/redfish/v1/Systems/system/PCIeDevices/pcie_card7/PCIeFunctions/0",
  "@odata.type": "#PCIeFunction.v1_2_3.PCIeFunction",
  "ClassCode": "0x020000",
  "DeviceId": "0x1657",
  "FunctionId": 0,
  "Id": "0",
  "Links": {
    "PCIeDevice": {
      "@odata.id": "/redfish/v1/Systems/system/PCIeDevices/pcie_card7"
    }
  },
  "Name": "PCIe Function",
  "RevisionId": "0x01",
  "SubsystemId": "0x0420",
  "SubsystemVendorId": "0x1014",
  "VendorId": "0x14e4"
}
```

Change-Id: I99add03210983a255a615512b6137d04982f83a5
Signed-off-by: Lakshmi Yadlapati <lakshmiy@us.ibm.com>