ARM: multi_v7_defconfig: Enable exynos-gsc driver as module

Exynos5 SoCs have a General SCALER (GSCALER) IP block that can be used
to do video streams scaling and color space conversions by hardware.
Enable support for its driver as a module so the GSCALER can be tested.

Signed-off-by: Javier Martinez Canillas <javier@osg.samsung.com>
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>