Add auxiliary device driver for mlx5 subfunction auxiliary device.

A mlx5 subfunction is similar to PCI PF and VF. For a subfunction
an auxiliary device is created.

As a result, when mlx5 SF auxiliary device binds to the driver,
its netdev and rdma device are created, they appear as

$ ls -l /sys/bus/auxiliary/devices/
mlx5_core.sf.4 -> ../../../devices/pci0000:00/0000:00:03.0/0000:06:00.0/mlx5_core.sf.4

$ ls -l /sys/class/net/eth1/device
/sys/class/net/eth1/device -> ../../../mlx5_core.sf.4

$ cat /sys/bus/auxiliary/devices/mlx5_core.sf.4/sfnum
88

$ devlink dev show
pci/0000:06:00.0
auxiliary/mlx5_core.sf.4

$ devlink port show auxiliary/mlx5_core.sf.4/1
auxiliary/mlx5_core.sf.4/1: type eth netdev p0sf88 flavour virtual port 0 splittable false

$ rdma link show mlx5_0/1
link mlx5_0/1 state ACTIVE physical_state LINK_UP netdev p0sf88

$ rdma dev show
8: rocep6s0f1: node_type ca fw 16.29.0550 node_guid 248a:0703:00b3:d113 sys_image_guid 248a:0703:00b3:d112
13: mlx5_0: node_type ca fw 16.29.0550 node_guid 0000:00ff:fe00:8888 sys_image_guid 248a:0703:00b3:d112

In future, devlink device instance name will adapt to have sfnum
annotation using either an alias or as devlink instance name described
in RFC [1].

[1] https://lore.kernel.org/netdev/20200519092258.GF4655@nanopsycho/Signed-off-by: Parav Pandit <parav@nvidia.com>
Reviewed-by: Vu Pham <vuhuong@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>