Change cache policy prefix, add alloc policy and remove incoherent
Change cache policy prefix, add alloc policy and remove incoherent

- Change prefix to pma_cache_write for cache write policies
- Add pma_cache_alloc policies (combine for 4 combinations)
- Remove pma_policy_incoherent and add note about non-presence
- Remove pma_amo_none and add note about non-presence
- Change pma_amo prefix to pma_io_amo; add note about main memory
- Add pma_io_size flags for 32,64,128; {256,512,1024}-bit
- Adjust numbering and fix indentation (now fills a 32-bit word)

rv8-io/rv8,rv8-io/rv8,rv8-io/rv8