milkymist-softusb: Don't map RAM memory regions in the device itself

Don't map the pmem and dmem RAM memory regions in the milkymist-softusb
device itself. Instead just expose them as sysbus mmio regions which
the device creator can map appropriately. This allows us to drop the
pmem_base and dmem_base properties. Instead of going via
cpu_physical_memory_read/_write when the device wants to access the
RAMs, we just keep a host pointer to the memory and use that.

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: 1363358063-23973-5-git-send-email-peter.maydell@linaro.org
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
