kernel: Add K_DELAYED_WORK_DEFINE

Adds a K_DELAYED_WORK_DEFINE, matching the K_WORK_DEFINE macro, with
accompanying Z_DELAYED_WORK_INITIALIZER macro.

Makes k_delayed_work_init a static inline function, like its K_WORK
counterpart.

Signed-off-by: Trond Einar Snekvik <Trond.Einar.Snekvik@nordicsemi.no>