i2c: gpio: Add support for deferred probing

GPIOs may not be available immediately when i2c-gpio looks for them.
Implement support for deferred probing so that probing can be
attempted again later when GPIO pins are finally available.

Signed-off-by: Jean Delvare <khali@linux-fr.org>
Tested-by: Bo Shen <voice.shen@atmel.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>