xlnx-zynqmp: Add support for high DDR memory regions

xlnx-zynqmp: Add support for high DDR memory regions

The Xilinx ZynqMP SoC and EP108 board supports three memory regions:
 - A 2GB region starting at 0
 - A 32GB region starting at 32GB
 - A 256GB region starting at 768GB

This patch adds support for the first two memory regions, which is
automatically created based on the size specified by the QEMU memory
command line argument.

On hardware the physical memory region is one continuous region, it is then
mapped into the three different regions by the DDRC. As we don't model the
DDRC this is done at startup by QEMU. The board creates the memory region and
then passes that memory region to the SoC. The SoC then maps the memory
regions.

Signed-off-by: Alistair Francis <alistair.francis@xilinx.com>
Reviewed-by: Peter Crosthwaite <crosthwaite.peter@gmail.com>
Message-id: a1e47db941d65733724a300fcd98b74fbeeaaf22.1452637205.git.alistair.francis@xilinx.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>