Add support for "i2c-mux-idle-disconnect" binding to match the
behavior, already present in upstream kernel driver. This feature is a
must have for I2C bus topologies with multiple muxes connected to
child segments containing slaves with identical addresses (e.g. ZII's
SCU4)

Signed-off-by: Andrey Smirnov <andrew.smirnov@gmail.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>