mfd: max77686: Use module_i2c_driver() instead of subsys initcall

The driver's init and exit function don't do anything besides adding and
deleting the I2C driver so the module_i2c_driver() macro could be used.

Currently is not being used because the driver is initialized at subsys
initcall level, claiming that this is done to allow consumers devices to
use the resources provided by this driver. But dependencies are in DT so
manual ordering of init calls is not necessary any more.

Signed-off-by: Javier Martinez Canillas <javier@osg.samsung.com>
Reviewed-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Reviewed-by: Andi Shyti <andi.shyti@samsung.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>