MIPS: PCI: Add a hook for IORESOURCE_BUS in pci_controller/bridge_controller

On SGI Origin 2k/Onyx2 and SGI Octane systems, there can exist multiple PCI
buses attached to the Xtalk bus.  The current code will stop counting PCI buses
after it finds the first one.  If one installs the optional PCI cardcage
("shoebox") into these systems, because of the order of the Xtalk widgets, the
current PCI code will find the cardcage first, and fail to detect the BaseIO
PCI devices, which are on a higher Xtalk widget ID.

This patch adds the hooks needed for resolving this issue in the IP27 PCI code
(in a later patch).

Verified on both an SGI Onyx2 and an SGI Octane.

Signed-off-by: Joshua Kinard <kumba@gentoo.org>
Cc: Linux MIPS List <linux-mips@linux-mips.org>
Patchwork: https://patchwork.linux-mips.org/patch/9074/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>