NTB: add pci shutdown handler for AMD NTB

The PCI shutdown handler is invoked in response
to system reboot or shutdown. A data transfer
might still be in flight when this happens. So
the very first action we take here is to send
a link down notification, so that any pending
data transfer is terminated. Rest of the actions
are same as that of PCI remove handler.

Signed-off-by: Arindam Nath <arindam.nath@amd.com>
Signed-off-by: Jon Mason <jdmason@kudzu.us>