ARM: exynos: introduce EXYNOS_ATAGS symbol

As a preparation for multiplatform support, this introduces
a new Kconfig symbol to split the ATAGS based EXYNOS platforms
from the DT based ones. Turning off CONFIG_EXYNOS_ATAGS disables
all platforms that are not yet converted to DT, and we can
have code that relies on DT checking for this symbol being
disabled.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>