Allow Mellanox switch devices to be configured if only I2C bus is set

Mellanox switches (mlxsw) supports I2C systems without PCI, in order to
give the ability to the users to use such functionality, there is need
to update Kconfig.

Signed-off-by: Ohad Oz <ohado@mellanox.com>
Acked-by: Leon Romanovsky <leonro@mellanox.com>
Acked-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>