i2c: i2c-mux-gpio: convert to use an explicit i2c mux core

Allocate an explicit i2c mux core to handle parent and child adapters
etc. Update the select/deselect ops to be in terms of the i2c mux core
instead of the child adapter.

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