Use dma_addr_t type for scatter/gather code

Use dma_addr_t type for scatter/gather code

This patch uses the newly created dma_addr_t type throughout the
scatter/gather handling code in dma-helpers.c whenever we need to
represent a dma bus address.  This makes a better distinction as to
what is a bus address and what is a cpu physical address.  Since we
don't support IOMMUs yet, they can't be very different for now, but
that will change in future, and this preliminary helps clarify what's
going on.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>