mlx5: Create an auxiliary device for fwctl_mlx5

If the device supports User Context then it can support fwctl. Create an
auxiliary device to allow fwctl to bind to it.

Create a sysfs like:

$ ls /sys/devices/pci0000:00/0000:00:0a.0/mlx5_core.fwctl.0/driver -l
lrwxrwxrwx 1 root root 0 Apr 25 19:46 /sys/devices/pci0000:00/0000:00:0a.0/mlx5_core.fwctl.0/driver -> ../../../../bus/auxiliary/drivers/mlx5_fwctl.mlx5_fwctl

Link: https://patch.msgid.link/r/[e-mail]
Signed-off-by: Saeed Mahameed <[e-mail]>
Acked-by: Leon Romanovsky <[e-mail]>
Signed-off-by: Jason Gunthorpe <[e-mail]>

drivers/net/ethernet/mellanox/mlx5/core/dev.c

···
 	MLX5_INTERFACE_PROTOCOL_VNET,
 
 	MLX5_INTERFACE_PROTOCOL_DPLL,
+	MLX5_INTERFACE_PROTOCOL_FWCTL,
 };
+
+static bool is_fwctl_supported(struct mlx5_core_dev *dev)
+{
+	/* fwctl is most useful on PFs, prevent fwctl on SFs for now */
+	return MLX5_CAP_GEN(dev, uctx_cap) && !mlx5_core_is_sf(dev);
+}
 
 static const struct mlx5_adev_device {
 	const char *suffix;
···
 					   .is_supported = &is_mp_supported },
 	[MLX5_INTERFACE_PROTOCOL_DPLL] = { .suffix = "dpll",
 					   .is_supported = &is_dpll_supported },
+	[MLX5_INTERFACE_PROTOCOL_FWCTL] = { .suffix = "fwctl",
+					    .is_supported = &is_fwctl_supported },
 };
 
 int mlx5_adev_idx_alloc(void)