drivers/nvme: Add command infrastructure

Based on FreeBSD's implementation made by James Harris, Intel Copyright
2012-2016.

This is the corner stone of the whole NVMe logic: sending commands and
getting replies, all through memory shared from the host to the
controller.

Then using it to inialize admit/IO queues and identifying the
controller.

Signed-off-by: Tomasz Bursztyka <tomasz.bursztyka@linux.intel.com>