lib: os: spsc_pbuf: Add option to use cache

Add flags option to init call and a flag to use cache.
Add Kconfig choice to pick how to approach cache. Cache can be
enforced in all spsc_pbuf instances, disable in all, or runtime selected
based on configuration flag. Option is added to allow memory footprint
savings.

Signed-off-by: Krzysztof Chruscinski <krzysztof.chruscinski@nordicsemi.no>