nvme: split command submission helpers out of pci.c

nvme: split command submission helpers out of pci.c

Create a new core.c and start by adding the command submission helpers
to it, which are already abstracted away from the actual hardware queues
by the block layer.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Acked-by: Keith Busch <keith.busch@intel.com>
Signed-off-by: Keith Busch <keith.busch@intel.com>
Signed-off-by: Jens Axboe <axboe@fb.com>
(cherry picked from commit 21d34711e1b5970acfb22bddf1fefbfbd7e0123b)

Orabug: 25130845
Conflicts:
        drivers/nvme/host/Makefile
        drivers/nvme/host/nvme.h
        drivers/nvme/host/pci.c

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