ARM: EXYNOS: Make combiner_of_init function static

Silences the following warning:
arch/arm/mach-exynos/common.c:576:12: warning:
symbol 'combiner_of_init' was not declared. Should it be static?

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