mfd: max77686: Allow driver to be built as a module

The driver's Kconfig symbol is a boolean but nothing prevents the driver
to be built as a module instead of built-in. It is true that most system
integrators will choose the latter but the config should not restrict it.

Signed-off-by: Javier Martinez Canillas <javier@osg.samsung.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>