i2c: move OF helpers into the core

I2C of helpers used to live in of_i2c.c but experience (from SPI) shows
that it is much cleaner to have this in the core. This also removes a
circular dependency between the helpers and the core, and so we can
finally register child nodes in the core instead of doing this manually
in each driver. So, fix the drivers and documentation, too.

Signed-off-by: Wolfram Sang <wsa@the-dreams.de>

drivers/i2c/busses/i2c-gpio.c

@@ -16,7 +16,6 @@
 #include <linux/platform_device.h>
 #include <linux/gpio.h>
 #include <linux/of_gpio.h>
-#include <linux/of_i2c.h>
 
 struct i2c_gpio_private_data {
 	struct i2c_adapter adap;
@@ -224,8 +223,6 @@ static int i2c_gpio_probe(struct platform_device *pdev)
 	if (ret)
 		goto err_add_bus;
 
-	of_i2c_register_devices(adap);
-
 	platform_set_drvdata(pdev, priv);
 
 	dev_info(&pdev->dev, "using pins %u (SDA) and %u (SCL%s)\n",