mlxsw: spectrum: Push RALUE packing and writing into low-level router ops

With follow-up introduction of XM implementation, XMDR register is
going to be optionally used instead of RALUE register. Push the RALUE
packing helpers and write call into low-level router ops.

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