ARM: s3c64xx: multiplatform support

After all preparation work is done, we can finally move the Kconfig
option for s3c64xx into ARCH_MULTIPLATFORM. This implies allowing
SAMSUNG_ATAGS for multiplatform again, but now disallowing the
ADC driver below it, as that still has dependencies on header files.

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