Move the x86 PMEM API implementation out of asm/cacheflush.h and into
its own header asm/pmem.h. This will allow members of the PMEM API to
be more easily identified on this and other architectures.
Signed-off-by: Ross Zwisler <ross.zwisler@linux.intel.com> Suggested-by: Christoph Hellwig <hch@lst.de> Reviewed-by: Christoph Hellwig <hch@lst.de> Signed-off-by: Dan Williams <dan.j.williams@intel.com>