lib: os: spsc_pbuf: Extend API with zero copy model

Added functions which allow to use zero copy model for handling
data within the packet buffer.

Additionally, added handling of cache by adding option to keep rd_idx
in different cache line than wr_idx and data.

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