ARM: make ARCH_S3C24XX select USE_OF and clean-up boot/dts/Makefile

ARCH_S3C24XX is a very exceptional platform that some DT files in
arch/arm/boot/dts/, but does not select USE_OF.

All the other platforms with DT files correctly select USE_OF
directly or indirectly (Most of them are either ARCH_MULTIPLATFORM
or ARM_SINGLE_ARMV7M).

With ARCH_S3C24XX fixed, "ifeq ($(CONFIG_OF),y)" in DT Makefile
can be deleted.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>