ARM: pci: add ->add_bus() and ->remove_bus() hooks to hw_pci

Some PCI drivers may need to adjust the pci_bus structure after it has
been allocated by the Linux PCI core. The PCI core allows
architectures to implement the pcibios_add_bus() and
pcibios_remove_bus() for this purpose. This commit therefore extends
the hw_pci and pci_sys_data structures of the ARM PCI core to allow
PCI drivers to register ->add_bus() and ->remove_bus() in hw_pci,
which will get called when a bus is added or removed from the system.

This will be used for example by the Marvell PCIe driver to connect a
particular PCI bus with its corresponding MSI chip to handle Message
Signaled Interrupts.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Reviewed-by: Thierry Reding <thierry.reding@gmail.com>
Acked-by: Russell King <rmk+kernel@arm.linux.org.uk>
Tested-by: Daniel Price <daniel.price@gmail.com>
Tested-by: Thierry Reding <thierry.reding@gmail.com>
Signed-off-by: Jason Cooper <jason@lakedaemon.net>