parisc: Add qemu fw_cfg interface

When running on qemu, SeaBIOS-hppa stores the iomem address for the
emulated fw_cfg port in PAGE0_>pad0[2/3]. Let the Linux driver
auto-configure the fw_cfg interface with it, so that the fw_cfg info
shows up in /sys/firmware/qemu_fw_cfg.

Signed-off-by: Helge Deller <deller@gmx.de>