microblaze/PCI: Add multidomain support for procfs

We create a procfs directory for every PCI bus.  Previously, the directory
name was just the bus number, so using the same bus number in different
domains caused a kernel crash when we tried to create a duplicate
directory.

Make pci_proc_domain() return the domain number, so procfs directories for
buses in domain 0 are named with just the bus number, and directories for
buses in other domains include both the domain number and the bus number.

[bhelgaas: changelog]
Signed-off-by: Bharat Kumar Gogada <bharatku@xilinx.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Michal Simek <michal.simek@xilinx.com>