ARM: highbank: enable DMA zone for LPAE

Some devices are restricted to 32-bit DMA. Thus the platform dma_zone_size
needs to be set. Otherwise dma-mapping code is complaining, e.g.

calxedaxgmac fff50000.ethernet: coherent DMA mask 0xffffffff is smaller
than system GFP_DMA mask 0xffffffffffffffff

Signed-off-by: Andreas Herrmann <andreas.herrmann@calxeda.com>
Signed-off-by: Rob Herring <rob.herring@calxeda.com>