ARM: S5P64X0: Add S5P64X0(S5P6440 and S5P6450) initialization support

This patch adds ARCH_S5P64X0 which can support S5P6440 and S5P6450 with
one kernel image. So moved some files of mach-s5p6440 into the new ARCH
directory mach-s5p64x0.

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