staging/hfi1: Add function stubs for TID caching

Add mmu notify helper functions and TID caching function
stubs in preparation for the TID caching implementation.

TID caching makes use of the MMU notifier to allow the driver
to respond to the user freeing memory which is allocated to
the HFI.

This patch implements the basic MMU notifier functions to insert,
find and remove buffer pages from memory based on the mmu_notifier
being invoked.

In addition it places stubs in place for the main entry points by
follow on code.

Follow up patches will complete the implementation of the interaction
with user space and makes use of these functions.

Signed-off-by: Mitko Haralanov <mitko.haralanov@intel.com>
Signed-off-by: Ira Weiny <ira.weiny@intel.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>