mlxsw: Introduce support for I2C bus

Add I2C bus implementation for Mellanox Technologies Switch ASICs.
This includes command interface implementation using input / out mailboxes,
whose location is retrieved from the firmware during probe time.

Signed-off-by: Vadim Pasternak <vadimp@mellanox.com>
Reviewed-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>