ARM: s3c: Cleanup from old plat-samsung include

The arch/arm/plat-samsung/ was removed and merged into
arch/arm/mach-s3c/ so the include path is not needed anymore.

Fixes: 71b9114d ("ARM: s3c: move into a common directory")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Link: https://lore.kernel.org/r/20200821074251.26798-1-krzk@kernel.org