dma: Define dma_context_memory and use in sysbus-ohci

dma: Define dma_context_memory and use in sysbus-ohci

Define a new global dma_context_memory which is a DMAContext corresponding
to the global address_space_memory AddressSpace. This can be used by
sysbus peripherals like sysbus-ohci which need to do DMA.

In particular, use it in the sysbus-ohci device, which fixes a
segfault when attempting to use that device.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Peter Crosthwaite <peter.crosthwaite@xilinx.com>