powerpc/pci: Add pci_find_controller_for_domain()

Add a helper to find the pci_controller structure based on the domain
number / phb id.

Signed-off-by: Oliver O'Halloran <oohall@gmail.com>
Reviewed-by: Sam Bobroff <sbobroff@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>