MAINTAINERS: list drivers/acpi/nfit/ files for libnvdimm sub-system

Patches that update the drivers/acpi/nfit/ directory need to be copied
to the nvdimm mailing list. The drivers/acpi/nfit* glob has been broken
ever since the nfit driver source was refactored into multiple files
under the drivers/acpi/nfit/ directory.

Reported-by: Prarit Bhargava <prarit@redhat.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>