mlxsw: core: Add registration APIs for system event handler

The purpose of system event handler is to handle system interrupts.
Such interrupts are raised to CPU from system programmable logic
devices, upon specific system wide changes, like line card activation
and deactivation.

The purpose is to create an alternative to trap mechanism, which
delivers these events to driver over PCI bus, but not available for
the driver working over I2C bus.

Mechanism is system dependent and applicable only for the systems
equipped with programmable devices with custom logic.

Add APIs for event handler registration and un-registration and API
which should be invoked from the registered callbacks when system
interrupt is raised to CPU.

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>