hw/core: Add a config switch for the "split-irq" device

The "split-irq" device is currently only used by machines that use
CONFIG_ARMSSE. Let's add a proper CONFIG_SPLIT_IRQ switch for this
so that it only gets compiled when we really need it.

Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20190817101931.28386-7-thuth@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>