Memory mapped serial device is in fact a sysbus device. The following
patches will make use of sysbus facilities for resource and
registration. In particular, "serial-mm: use sysbus facilities" will
move internal serial realization to serial_mm_realize callback to
follow qdev best practices.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>