devicetree: bindings: gpio-i2c belongs to i2c not gpio

gpio-i2c describes an I2C controller (using gpios for data and clock),
so it must be described in i2c, not gpio.
Signed-off-by: Wolfram Sang <w.sang@pengutronix.de>
Acked-by: Rob Herring <rob.herring@calxeda.com>