mips: use sysbus_mmio_get_region() instead of internal fields

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>