Add the NvmeBarRegs enum and use these instead of explicit register
offsets.

Signed-off-by: Klaus Jensen <k.jensen@samsung.com>
Reviewed-by: Gollu Appalanaidu <anaidu.gollu@samsung.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Keith Busch <kbusch@kernel.org>