Add a 'dma' property allowing machine creation to provide the address-space
SDHCI DMA operates on.

[based on a patch from Alistair Francis <alistair.francis@xilinx.com>
 from qemu/xilinx tag xilinx-v2016.1]
Signed-off-by: NPhilippe Mathieu-Daudé <f4bug@amsat.org>
Message-id: 20180115182436.2066-15-f4bug@amsat.org
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>