ARM: s5pc, exynos: move Samsung ARM SoC specific code under arch/arm/

This patch moves S5PC, EXYNOS specific directory entries
from the toplevel Makefile to arch/arm/cpu/armv7/Makefile
using Kbuild descending feature.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Cc: Minkyu Kang <mk7.kang@samsung.com>