i2c: mux: inform the i2c mux core about how it is used

The i2c mux core can then take appropriate action depending on if it is
used for an actual i2c mux, for a gate or for an arbitrator (the last
is the case for these drivers). This adds support for the new clearer
and more compact devicetree bindings that was added recently.

Reviewed-by: Wolfram Sang <wsa@the-dreams.de>
Signed-off-by: Peter Rosin <peda@axentia.se>