Refactor SystemPCIeFunctionCollection method

Move SystemPCIeFunctionCollection 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",
  "@odata.type": "#PCIeFunctionCollection.PCIeFunctionCollection",
  "Description": "Collection of PCIe Functions for PCIe Device pcie_card7",
  "Members": [
    {
      "@odata.id": "/redfish/v1/Systems/system/PCIeDevices/pcie_card7/PCIeFunctions/0"
    }
  ],
  "Name": "PCIe Function Collection",
  "PCIeFunctions@odata.count": 1
}
```

Change-Id: I5aa10ce0b4d2f20104612f840cf4098698a83470
Signed-off-by: Lakshmi Yadlapati <lakshmiy@us.ibm.com>