mlxsw: reg: Add Management Cable IO and Notifications register

Add the Management Cable IO and Notifications register. It will be used
to retrieve the power mode status of a module in subsequent patches and
whether a module is present in a cage or not.

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