ARM: exynos: Make ARCH_EXYNOS3 a default option

By default for ARMv7 Exynos platform we select all flavors.  One kernel
image simplifies testing and maintenance.  However Exynos3 was not
selected by default so far (thus it was not present in multi_v7
kernel).

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>