ARM: s3c64xx: Drop unneeded select of TIMER_OF

Support for Samsung S3C64XX systems depends on ARCH_MULTI_V6, and thus
on ARCH_MULTIPLATFORM.
As the latter selects TIMER_OF, there is no need for MACH_S3C64XX_DT to
select TIMER_OF.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>