ARM: S5PV210: Fix build warning

Fixed the following warning for S5PV210.

arch/arm/mach-s5pv210/pm.c: In function 's5pv210_pm_add':
arch/arm/mach-s5pv210/pm.c:139: warning: assignment from
incompatible pointer type

Also, staticized the function.

Signed-off-by: Abhilash Kesavan <a.kesavan@samsung.com>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>