mlxsw: reg: Add Ports Mapping Event Configuration Register

The PMECR register is used to enable/disable event triggering
in case of local port mapping change.

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