milkymist-minimac2: Just expose buffers as a sysbus mmio region

Just expose the register buffers memory as a standard sysbus mmio
region which the creator of the device can map, rather than
providing a qdev property which the creator has to set to the
base address and then doing the mapping in the device's own
init function.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Andreas Färber <afaerber@suse.de>
Acked-by: Michael Walle <michael@walle.cc>
Message-id: 1363358063-23973-4-git-send-email-peter.maydell@linaro.org
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
