Better support for dma_addr_t variables

Better support for dma_addr_t variables

A while back, we introduced the dma_addr_t type, which is supposed to
be used for bus visible memory addresses.  At present, this is an
alias for target_phys_addr_t, but this will change when we eventually
add support for guest visible IOMMUs.

There are some instances of target_phys_addr_t in the code now which
should really be dma_addr_t, but can't be trivially converted due to
missing features which this patch corrects.

 * We add DMA_ADDR_BITS analagous to TARGET_PHYS_ADDR_BITS.  This is
   important where we need to make a compile-time (#if) based on the
   size of dma_addr_t.

 * We add a new helper macro to create device properties which take a
   dma_addr_t, currently an alias to DEFINE_PROP_TADDR().

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>