[LIBS] Allow custom memory flags when creating mempool

We should be able to create mempool with different memory flags
based on requirement of mempool user. For instance one may want
to create a mempool of objects from a dma-coherent memory.

This patch updates mempool_create() API enabling us to pass
memory flags for the pool of objects.

Signed-off-by: Anup Patel <anup@brainfault.org>