hw/pci-host/piix: Move i440FX declarations to hw/pci-host/i440fx.h

The hw/pci-host/piix.c contains a mix of PIIX3 and i440FX chipsets
functions. To be able to split it, we need to export some
declarations first.

Reviewed-by: Aleksandar Markovic <amarkovic@wavecomp.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>