hw/usb: Extract VT82C686 UHCI PCI function into a new unit

Extract the VT82C686 PCI UHCI function into a new unit so
it is only build when the VT82C686 south bridge is selected.

Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <20210309190802.830969-4-f4bug@amsat.org>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>