i2c: i2c_gpio: keep probe resident for hotplugged devices.

Change the i2c_gpio driver to use platform_driver_register()
instead of platform_driver_probe() to ensure that is can
attach to any devices that may be loaded after it has initialised.

Acked-by: Haavard Skinnemoen <haavard.skinnemoen@atmel.com>
Signed-off-by: Ben Dooks <ben-linux@fluff.org>