net/devlink: Add E-Switch mode control

Add the commands to set and show the mode of SRIOV E-Switch, two modes
are supported:

* legacy: operating in the "old" L2 based mode (DMAC --> VF vport)

* switchdev: the E-Switch is referred to as whitebox switch configured
using standard tools such as tc, bridge, openvswitch etc. To allow
working with the tools, for each VF, a VF representor netdevice is
created by the E-Switch manager vendor device driver instance (e.g PF).

Signed-off-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>