drivers/nvme: Add namespace support

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

Namespace in this context, will be a disk. It's not exposed from DTS, as
an actualy NVMe hardware controller card can bring more than one
namespace (disk).

Thus namespace are not instanciated through the device driver model, but
statically allocated and runtime configured, depending on what the
controller exposes.

By default the amount of namespace supported is one as it is the most
common setup.

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