i2c-mux: Add support for device auto-detection

Let I2C bus segments behind multiplexers have a class. This allows for
device auto-detection on these segments. As long as parent segments
don't share the same class, it should be fine.

I implemented support in drivers i2c-mux-gpio and i2c-mux-pca954x. I
left i2c-mux-pca9541 and i2c-mux-pinctrl alone for the moment as I
don't know if this feature makes sense for the use cases of these
drivers.

Signed-off-by: Jean Delvare <khali@linux-fr.org>
Cc: Peter Korsgaard <peter.korsgaard@barco.com>
Cc: David Daney <david.daney@cavium.com>
Cc: Michael Lawnick <ml.lawnick@gmx.de>
Cc: Rodolfo Giometti <giometti@linux.it>

drivers/i2c/muxes/i2c-mux-pinctrl.c

@@ -221,7 +221,7 @@ static int __devinit i2c_mux_pinctrl_probe(struct platform_device *pdev)
 				(mux->pdata->base_bus_num + i) : 0;
 
 		mux->busses[i] = i2c_add_mux_adapter(mux->parent, &pdev->dev,
-						     mux, bus, i,
+						     mux, bus, i, 0,
 						     i2c_mux_pinctrl_select,
 						     deselect);
 		if (!mux->busses[i]) {