mlxsw: reg: Add Port Module Type Mapping register

Add the Port Module Type Mapping (PMTP) register. It will be used by
subsequent patches to query port module types and forbid certain
configurations based on the port module's type.

Signed-off-by: Danielle Ratson <danieller@nvidia.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>