kernel code reading: register a service driver on a pcie device

A pcie port could support several services, the service driver is registered
by calling pcie_port_service_register().

One example is the pci hotplug service, in pcied_init().
kernel code reading: device_register()

When a device is found in kernel, device_register will be called.

After the device is initialized, bus_probe_device will be called to probe the
proper driver for this device.
kernel code reading: create pcie_express bus and pcie_device

Driver "pcieport" is a PCI driver, who will create the "pci_express" bus and
create those pcie_device devices.

As the document says, those pcie_device represents a service of the pcie
device. That is very interesting.

API pcie_port_service_register() is used to register the drivers for those
pcie_device.