nvme: use offset instead of a struct for registers

nvme: use offset instead of a struct for registers

This makes life easier for future non-PCI drivers where access to the
registers might be more complicated.  Note that Linux drivers are
pretty evenly split between the two versions, and in fact the NVMe
driver already uses offsets for the doorbells.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Acked-by: Keith Busch <keith.busch@intel.com>
[Fixed CMBSZ offset]
Signed-off-by: Keith Busch <keith.busch@intel.com>
Signed-off-by: Jens Axboe <axboe@fb.com>
(cherry picked from commit 7a67cbea653e444d04d7e850ab9631a14a196422)

Conflicts:
        Merge conflict due to the Samsung Errata patch
        drivers/nvme/host/pci.c

Signed-off-by: Ashok Vairavan <ashok.vairavan@oracle.com>
Reviewed-by: Martin K. Petersen <martin.petersen@oracle.com>