MAINTAINERS: add PCI Endpoint NTB drivers to NTB files

The PCI Endpoint NTB drivers are under the NTB umbrella.  Add an entry
there to allow for notification of changes for it.

Signed-off-by: Jon Mason <jdmason@kudzu.us>
Acked-by: Bjorn Helgaas <bhelgaas@google.com>