pmem, x86: move x86 PMEM API to new pmem.h header

pmem, x86: move x86 PMEM API to new pmem.h header

Orabug: 22913653

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>
(cherry picked from commit 40603526569b304dd92f720f2f8ab11e828ea145)
Signed-off-by: Dan Duval <dan.duval@oracle.com>