mlxsw: reg: Add Port Module Plug/Unplug Event Register

PMPE register reports any operational status change of a module.
It will be used for enabling temperature warning event when a module is
plugged in.

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