mips: use sysbus_add_io()

mips: use sysbus_add_io()

sysbus devices should use the associated helper function instead of
calling memory_region_add_subregion() manually. This simplifies
slightly the code.

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>