mlxsw: Add interface to access registers and process events

Ethernet Management Datagrams (EMADs) are Ethernet packets sent between
the host and the device in order to configure the available device registers.
Another use case is notifications sent from the device to the host,
letting it know about certain events, such as port up / down.

Add the ability to construct EMADs with provisions to construct and
parse the registers' payloads. Implement EMAD transaction layer
which is responsible for the reliable transmission of EMADs. Also, add
an infrastructure used by the switch driver to register for particular
events generated by the device.

Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: Elad Raz <eladr@mellanox.com>
Reviewed-by: Scott Feldman <sfeldma@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>