[media] media: s5p-cec: Remove references to non-existent PLAT_S5P symbol

Commit d78c16cc ("ARM: SAMSUNG: Remove remaining legacy code")
removed the Kconfig symbol PLAT_S5P.
This patch remove the last occurrence of this symbol.
Signed-off-by: Corentin Labbe <clabbe.montjoie@gmail.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>

drivers/staging/media/s5p-cec/Kconfig

 config VIDEO_SAMSUNG_S5P_CEC
        tristate "Samsung S5P CEC driver"
-       depends on VIDEO_DEV && MEDIA_CEC_SUPPORT && (PLAT_S5P || ARCH_EXYNOS || COMPILE_TEST)
+       depends on VIDEO_DEV && MEDIA_CEC_SUPPORT && (ARCH_EXYNOS || COMPILE_TEST)
        ---help---
          This is a driver for Samsung S5P HDMI CEC interface. It uses the
          generic CEC framework interface.