Currently our PL080/PL081 model uses a combination of the CPU's
address space (via cpu_physical_memory_{read,write}()) and the
system address space for performing DMA accesses.

For the PL081s in the MPS FPGA images, their DMA accesses
must go via Master Security Controllers. Switch the
PL080/PL081 model to take a MemoryRegion property which
defines its downstream for making DMA accesses.

Since the PL08x are only used in two board models, we
make provision of the 'downstream' link mandatory and convert
both users at once, rather than having it be optional with
a default to the system address space.
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>
Reviewed-by: NPhilippe Mathieu-Daudé <f4bug@amsat.org>