ARM: s5p64x0_defconfig: Update for support S5P6440 and S5P6450

This patch updates s5p64x0_defconfig and changes the name from
s5p6440_defconfig so that can support S5P6440 and S5P6450 with
one kernel.

Tested on SMDK6440(S5P6440) and SMDK6450(S5P6450).

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