ARM: S3C64XX: Kill CONFIG_PLAT_S3C64XX

CONFIG_PLAT_S3C64XX has been kept in place way too long since it was
marked as temporary in commit

110d85ac ARM: S3C64XX: Eliminate plat-s3c64xx

After fixing all users of it in previous patches, this patch finally
kills this temporary Kconfig entry.

Signed-off-by: Tomasz Figa <tomasz.figa@gmail.com>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>