ARM: EXYNOS: fix compilation error introduced due to common clock migration

The functions exynos4_clk_init and exynos4_clk_register_fixed_ext
are applicable only on Exynos4 non-dt platforms. But when building
Exynos5 platforms without including Exynos4 platforms, the following
errors show up.

arch/arm/mach-exynos/built-in.o: In function `exynos_init_time':
arch/arm/mach-exynos/common.c:446: undefined reference to `exynos4_clk_init'
arch/arm/mach-exynos/common.c:447: undefined reference to `exynos4_clk_register_fixed_ext'

Fix this compilation errors by marking these calls as Exynos4 specific.

Signed-off-by: Thomas Abraham <thomas.ab@samsung.com>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>