i2c/of: Automatically populate i2c mux busses from device tree data.

i2c/of: Automatically populate i2c mux busses from device tree data.

For 'normal' i2c bus drivers, we can call of_i2c_register_devices()
and have the device tree framework automatically populate the bus with
the devices specified in the device tree.

This patch adds a common code to the i2c mux framework to have the mux
sub-busses be populated by the of_i2c_register_devices() too.  If the
mux device has an of_node, we populate the sub-bus' of_node so that
the subsequent call to of_i2c_register_devices() will find the
corresponding devices.

It seemed better to put this logic in i2c_add_mux_adapter() rather
than the individual mux drivers, as they will all probably want to do
the same thing.

Signed-off-by: David Daney <david.daney@cavium.com>
Acked-by: Stephen Warren <swarren@wwwdotorg.org>
Tested-by: Lars-Peter Clausen <lars@metafoo.de>
[wsa: removed superfluous ret-variable and fixed a typo in a comment]
Signed-off-by: Wolfram Sang <w.sang@pengutronix.de>