hw/isa: Introduce a CONFIG_ISA_SUPERIO switch for isa-superio.c

hw/isa: Introduce a CONFIG_ISA_SUPERIO switch for isa-superio.c

Currently, isa-superio.c is always compiled as soon as CONFIG_ISA_BUS
is enabled. But there are also machines that have an ISA BUS without
any of the superio chips attached to it, so we should not compile
isa-superio.c in case we only compile a QEMU for such a machine.
Thus add a proper CONFIG_ISA_SUPERIO switch so that this file only gets
compiled when we really, really need it.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>