nvme.h: add dword 12 - 15 fields to struct nvme_features

Signed-off-by: Arnav Dawn <a.dawn@samsung.com>
[hch: split from a larger patch, new changelog]
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Keith Busch <keith.busch@intel.com>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Reviewed-by: Johannes Thumshirn <jthumshirn@suse.de>