dt-bindings: i2c: mux: ltc4306: Add dt-bindings for I2C multiplexer/switch

This patch adds support for the Analog Devices / Linear Technology
LTC4306 and LTC4305 4/2 Channel I2C Bus Multiplexer/Switches.
The LTC4306 optionally provides two general purpose input/output pins
(GPIOs) that can be configured as logic inputs, opendrain outputs or
push-pull outputs via the generic GPIOLIB framework.

Signed-off-by: Michael Hennerich <michael.hennerich@analog.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Peter Rosin <peda@axentia.se>