i2c/gpio-i2cmux: Convert to use module_platform_driver()

Convert gpio-i2cmux to use the module_platform_driver() macro which
makes the code smaller and a bit simpler.

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Acked-by: Peter Korsgaard <peter.korsgaard@barco.com>
Signed-off-by: Jean Delvare <khali@linux-fr.org>