MIPS: RC32434: Define io_map_base for PCI controller

The code is rather based on trial-and-error than knowledge. Verified Via
Rhine functionality in PIO as well as MMIO mode.

[Looks sane -- Ralf]

Signed-off-by: Phil Sutter <n0-1@freewrt.org>
Tested-by: Florian Fainelli <florian@openwrt.org>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>