ARM: SAMSUNG: Fix missing s5p_init_cpu() declaration

The declaration of s5p_init_cpu() in arch/arm/mach-exynos/common.h
is not included in the platform file arch/arm/plat-samsung/cpu.c
which generates a warning.

Fix the following warning by moving the declaration to somewhere
both the machine and platform code can get to it, and including
the right files as necessary:

arch/arm/plat-samsung/cpu.c:47:13: warning: symbol 's5p_init_cpu' was not declared. Should it be static?

Signed-off-by: Ben Dooks <ben.dooks@codethink.co.uk>
Signed-off-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>