malloc: make alignment requirements more stringent

The external heaps API already implicitly expects start address
of the external memory area to be page-aligned, but it is not
enforced or documented. Fix this by implementing additional
parameter checks at memory add call, and document the page
alignment requirement explicitly.

Fixes: 7d75c31014f7 ("malloc: allow adding memory to named heaps")
Cc: stable@dpdk.org
Suggested-by: Yongseok Koh <yskoh@mellanox.com>
Signed-off-by: Anatoly Burakov <anatoly.burakov@intel.com>
Acked-by: Yongseok Koh <yskoh@mellanox.com>