sfc: add code to register and unregister encap matches

Add a hashtable to detect duplicate and conflicting matches.  If match
 is not a duplicate, call MAE functions to add/remove it from OR table.
Calling code not added yet, so mark the new functions as unused.

Signed-off-by: Edward Cree <ecree.xilinx@gmail.com>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Signed-off-by: David S. Miller <davem@davemloft.net>