Add the GPIO expander found on the i2c1 bus, behind the muxer to the
device tree.

This gpio expander will be used to get tachymeters values and data ready
interruptions from the nuvoton NAU7802 ADCs.
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>