mempool: rename addresses from physical to IOVA

The struct fields phys_addr_t rte_mempool_objhdr.physaddr and
rte_mempool_memhdr.phys_addr are renamed to rte_iova_t iova.
The deprecated names are kept in an anonymous union to avoid breaking
the API.

Signed-off-by: Thomas Monjalon <thomas@monjalon.net>
Acked-by: Santosh Shukla <santosh.shukla@caviumnetworks.com>
Acked-by: Olivier Matz <olivier.matz@6wind.com>