Clean up register definitions related to PCI Express Hot plug.

  - Add register definitions into include/linux/pci_regs.h, and use
    them instead of pciehp's locally definied register definitions.
  - Remove pciehp's locally defined register definitions
  - Remove unused register definitions in pciehp.
  - Some minor cleanups.
Signed-off-by: NKenji Kaneshige <kaneshige.kenji@jp.fujitsu.com>
Signed-off-by: NJesse Barnes <jbarnes@virtuousgeek.org>