FROMLIST: staging: vsoc: Create wc kernel mapping for region shm.

Map the region shm as write-combining instead of uncachable.

Signed-off-by: Alistair Strachan <astrachan@google.com>
[sent upstream via staging https://patchwork.kernel.org/patch/10376965/]
Bug: 702147120
Change-Id: If587b4f26abaa3580b889b04dbb655f8d5b4a8ee