hw/pci-host: Rename Raven ASIC PCI bridge as raven.c

The ASIC PCI bridge chipset from Motorola is named 'Raven'.
This chipset is used in the PowerPC Reference Platform (PReP),
but not restricted to it. Rename it accordingly.

Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Acked-by: David Gibson <david@gibson.dropbear.id.au>
Message-Id: <20210417103028.601124-5-f4bug@amsat.org>