ARM: s3c: remove s3c6400 support

No board file and no dts file references the s3c6400 now, it's only
s3c6410, so remove the final bits as well.

Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>