ipc: icmsg: Improve packets reception

This improves packet reception and fix an issue
where packets bigger than internal Rx buffer
were silently dropped. In current solution
local data coping for reception is not needed.
User gets direct pointer to shared memory which
allow to efficient receive as much data as was sent.

Signed-off-by: Kamil Gawor <Kamil.Gawor@nordicsemi.no>