eal/linux: select IOVA as VA mode for default case

eal/linux: select IOVA as VA mode for default case

When bus layer reports the preferred mode as RTE_IOVA_DC then
select the RTE_IOVA_VA mode:

- All drivers work in RTE_IOVA_VA mode, irrespective of physical
address availability.

- By default, a mempool asks for IOVA-contiguous memory using
RTE_MEMZONE_IOVA_CONTIG. This is slow in RTE_IOVA_PA mode and it
may affect the application boot time.

Signed-off-by: Jerin Jacob <jerinj@marvell.com>
Acked-by: Anatoly Burakov <anatoly.burakov@intel.com>
Signed-off-by: David Marchand <david.marchand@redhat.com>