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: NJean 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>

Update the MAINTAINERS entry and all other references accordingly.

Based on an original patch by Wolfram Sang.
Signed-off-by: NJean Delvare <khali@linux-fr.org>
Acked-by: NPeter Korsgaard <peter.korsgaard@barco.com>

[wsa: fixed merge conflict due to rework in i2c_add_mux_adapter()]
Signed-off-by: NWolfram Sang <w.sang@pengutronix.de>

Add an i2c mux driver providing access to i2c bus segments using a
hardware MUX sitting on a master bus and controlled through gpio pins.

E.G. something like:

  ----------              ----------  Bus segment 1   - - - - -
 |          | SCL/SDA    |          |-------------- |           |
 |          |------------|          |
 |          |            |          | Bus segment 2 |           |
 |  Linux   | GPIO 1..N  |   MUX    |---------------   Devices
 |          |------------|          |               |           |
 |          |            |          | Bus segment M
 |          |            |          |---------------|           |
  ----------              ----------                  - - - - -

SCL/SDA of the master I2C bus is multiplexed to bus segment 1..M
according to the settings of the GPIO pins 1..N.
Signed-off-by: NPeter Korsgaard <peter.korsgaard@barco.com>
Signed-off-by: NJean Delvare <khali@linux-fr.org>