mlxsw: spectrum_router: Add helpers for neighbor access

This is done as a preparation before introducing the ability to dump the
host table via dpipe, and to count the table size. The mlxsw's neighbor
representative struct stays private to the router module.

Signed-off-by: Arkadi Sharshevsky <arkadis@mellanox.com>
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>