settings: zms: add cache for linked list hash

Increase the load performance by adding an optional cache for the linked
list hashes.
This is used only when the settings_load is called with NULL parameter
and we need to load all Settings that exist in the persistent storage.
Cache is enabled using SETTINGS_ZMS_LL_CACHE and the size of the cache
is set using SETTINGS_ZMS_LL_CACHE_SIZE.
Each cache entry will add 8 bytes of RAM usage.

Signed-off-by: Riadh Ghaddab <rghaddab@baylibre.com>