mips: use sysbus_mmio_get_region() instead of internal fields

Register the memory region with sysbus_init_mmio() and look it up with
sysbus_mmio_get_region() to avoid accessing internal device fields.

Suggested-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Aleksandar Markovic <amarkovic@wavecomp.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>

hw/char/serial.c

@@ -998,6 +998,7 @@ static void serial_io_realize(DeviceState *dev, Error **errp)
     }
 
     memory_region_init_io(&s->io, NULL, &serial_io_ops, s, "serial", 8);
+    sysbus_init_mmio(SYS_BUS_DEVICE(sio), &s->io);
     sysbus_init_irq(SYS_BUS_DEVICE(sio), &s->irq);
 }