ARM: S5P64XX: Remove redundant selection PLAT_S5P.

This patch removes redundant selection PLAT_S5P in the CPU_S5P64XX config.
Because PLAT_S5P is selected if select ARCH_S5P64XX in the plat-s5p/Kconfig.

Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>