malloc: allow adding memory to named heaps

malloc: allow adding memory to named heaps

Add an API to add externally allocated memory to malloc heap. The
memory will be stored in memseg lists like regular DPDK memory.
Multiple segments are allowed within a heap. If IOVA table is
not provided, IOVA addresses are filled in with RTE_BAD_IOVA.

Signed-off-by: Anatoly Burakov <anatoly.burakov@intel.com>