mlxsw: spectrum: Introduce resource for getting offset of 4 lanes split port

In Spectrum-3 the modules have 8 lanes, so split by count 2 results in
two split ports each of 4 lanes. Add a resource that can be used to
obtain local port offset in that case.

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