gpio/samsung: make Kconfig options def_bool

The Samsung GPIO drivers are always built-in when the relevant
platform is selected.  Change the Kconfig symbol to def_bool y
dependant on the platform.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Acked-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>