mlxsw: core_linecards: Register a system event handler

Add line card system event handler. Register it with core. It is
triggered by system interrupts raised from chassis programmable logic
devices to CPU. The purpose is to handle line card state changes over
I2C bus.

Signed-off-by: Vadim Pasternak <vadimp@nvidia.com>
Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>