Update target repo for nvme patch contributions

Per http://www.nvmexpress.org/resources/linux-driver-information/

, the
old nvme git repo is stale.  Updating MAINTAINERS to the Supported
target currently used by the community.

Signed-off-by: Jay Freyensee <james_p_freyensee@linux.intel.com>

Updated by me to add Keith as the maintainer, me as the co-maintainer.

Signed-off-by: Jens Axboe <axboe@fb.com>