mlxsw: spectrum_router: Implement LPM trees management

Introduce basic LPM tree management allowing to share the trees in
between tables if the used prefixes in the tables are the same.
Build the tree structure according to the used prefixes. Although it is
not optimal for many use cases, this initial implementation does only
simple linear left-tree. More advanced structures will be introduced
later on, possibly including mechanisms to change trees on the fly.

Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Reviewed-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>