ARM: use phys_addr_t for DMA zone sizes

In order to specify a DMA zone size of 4GB on LPAE systems, the sizes need
to be 64-bit. So make machine_desc.dma_zone_size and arm_dma_zone_size be
phys_addr_t instead of unsigned long.

Signed-off-by: Rob Herring <rob.herring@calxeda.com>