Several MIPS platforms don't set pci_controller::io_map_base for their
PCI bridges.  This results in a panic in pci_iomap().  (The panic is
conditional on CONFIG_PCI_DOMAINS, but that is now enabled for all PCI
MIPS systems.)
Signed-off-by: NBen Hutchings <ben@decadent.org.uk>
Cc: linux-mips@linux-mips.org
Cc: Martin Michlmayr <tbm@cyrius.com>
Cc: Aurelien Jarno <aurelien@aurel32.net>
Cc: 584784@bugs.debian.org
Patchwork: https://patchwork.linux-mips.org/patch/1377/Signed-off-by: NRalf Baechle <ralf@linux-mips.org>