mlxsw: spectrum_router: Introduce XM implementation of router low-level ops

In order to offload entries to XM, implement a set of low-level
functions to work with LPM trees in XM and also to pack and write
FIB entries into XM.

Signed-off-by: Jiri Pirko <jiri@nvidia.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>