drivers/of: Add the of_find_i2c_device_by_node function.

The of_find_i2c_device_by_node function allows you to follow a
reference in the device tree to an i2c device node and then locate
the linux device instantiated by the device tree. Example use: an I2S
bus driver finding the i2c_device instance for a codec described by
a device tree node.

This was waiting for Anton's i2c patches that were just added.

Signed-off-by: Jon Smirl <jonsmirl@gmail.com>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>