mlxsw: Introduce Mellanox switch driver core

Add core components of Mellanox switch driver infrastructure.
Core infrastructure is designed so that it can be used by multiple
bus drivers (PCI now, I2C and SGMII are planned to be implemented
in the future). Multiple switch kind drivers can be registered as well.
This core serves as a glue between buses and drivers.

Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: Ido Schimmel <idosch@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>