PCI: xgene: Make explicitly non-modular

This code is not being built as a module by anyone:

  drivers/pci/host/Kconfig:config PCI_XGENE
  drivers/pci/host/Kconfig:       bool "X-Gene PCIe controller"

Remove uses of MODULE_DESCRIPTION(), MODULE_AUTHOR(), MODULE_LICENSE(),
etc., so that when reading the driver there is no doubt it is builtin-only.
The information is preserved in comments at the top of the file.

Replace module_platform_driver() with builtin_platform_driver(), which uses
the same init level priority, so init ordering is unchanged.

[bhelgaas: changelog]
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
CC: Tanmay Inamdar <tinamdar@apm.com>