media: sec: Remove PLAT_S5P dependency

The PLAT_S5P symbol was removed in commit d78c16cc ("ARM: SAMSUNG:
Remove remaining legacy code").

Remove the PLAT_S5P dependency from VIDEO_SAMSUNG_S5P_CEC.

Discovered with the
https://github.com/ulfalizer/Kconfiglib/blob/master/examples/list_undefined.py


script.

Signed-off-by: Ulf Magnusson <ulfalizer@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>