Define PMEM_CACHE_FLUSH

Change-Id: I98799c6f4e863a80048297fa1f95fac92bf78b05

Signed-off-by: Nathan Chan <e334cn@gmail.com>
pmem: Add cache flush ioctl for pmem buffers

based on: ade14d726615c9f008b82677068dd0ffae1e8d6a

Change-Id: I14dd45e0103e5b96182cd900072897103a9d782c

Signed-off-by: Nathan Chan <e334cn@gmail.com>