devlink: Introduce devlink port flavour virtual

Currently mlx5 PCI PF and VF devlink devices register their ports as
physical port in non-representors mode.

Introduce a new port flavour as virtual so that virtual devices can
register 'virtual' flavour to make it more clear to users.

An example of one PCI PF and 2 PCI virtual functions, each having
one devlink port.

$ devlink port show
pci/0000:06:00.0/1: type eth netdev ens2f0 flavour physical port 0
pci/0000:06:00.2/1: type eth netdev ens2f2 flavour virtual port 0
pci/0000:06:00.3/1: type eth netdev ens2f3 flavour virtual port 0

Reviewed-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: Parav Pandit <parav@mellanox.com>
Acked-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>