slist: Introduce CONTAINER macros

While iterating on each node element it is much more common to try to
access the container struct which up until now have been done manually
using CONTAINER_OF macro, so this introduce CONTAINER variants that allow
to iterate directly with container pointer rather than the list node.

Change-Id: Ia24d9b88d5e2c43ffd476f565faf5bb523a9927b
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>