Add definitive encoding for c-/xmem_req_type signal
Created by: moimfeld
I issue this PR as agreed in the bi-weekly cv-x-if meeting. See Meeting Minutes from 31.05.2021. This change adds a definitive encoding for the cmem_req_type and xmem_req_type signal, which is necessairy for the memory interface to be general and not core/accelerator specific.