[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>