i2c: Retain info->of_node in i2c_new_device()

Currently, of_i2c_register_devices() is responsible for retaining
info->of_node, but we're about to expose a function to parse I2C board
info without registering the I2C device.

We could possibly let this function retain ->of_node, but this approach
is prone to reference leak since people will have to remember to call
of_node_put() if something goes wrong between the OF node parsing and
the registration step.
Let's just retain the ->of_node in i2c_new_register() instead.

Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>