i2c: mux: Add dt support to i2c-mux-gpio driver

Allow the i2c-mux-gpio to be used by a device tree enabled device. The
bindings are inspired by the one found in the i2c-mux-pinctrl driver.

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Reviewed-by: Stephen Warren <swarren@nvidia.com>
Acked-by: Peter Korsgaard <peter.korsgaard@barco.com>
[wsa: fixed some whitespace]

Signed-off-by: Wolfram Sang <w.sang@pengutronix.de>