ARM: EXYNOS: Add ARCH_EXYNOS and reorganize arch/arm/mach-exynos

The arch/arm/mach-exynos4 directory (CONFIG_ARCH_EXYNOS4) has
made for plaforms based on EXYNOS4 SoCs. But since upcoming
Samsung's SoCs such as EXYNOS5 (ARM Cortex A15) can reuse most
codes in current mach-exynos4, one mach-exynos directory will
be used for them.

This patch changes to CONFIG_ARCH_EXYNOS (arch/arm/mach-exynos)
but keeps original CONFIG_ARCH_EXYNOS4 in mach-exynos/Kconfig to
avoid changing in driver side.

Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>