MAINTAINERS: Add backup maintainers for libnvdimm and DAX

Adding additional maintainers to libnvdimm related code and DAX.

Signed-off-by: Dave Jiang <dave.jiang@intel.com>
Acked-by: Ross Zwisler <ross.zwisler@linux.intel.com>
Acked-by: Vishal Verma <vishal.l.verma@intel.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>