ARM: EXYNOS: Enable multi-platform build support

This makes it possible to enable the Exynos platform as part of a
multiplatform kernel.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>