ARM: S3C24XX: let S3C2412_PM select S3C2412_PM_SLEEP

The code to enter sleep is used by both the s3c2412 and s3c2416 and
was thus factored out into an extra config option.

But it seems it was forgotten to add the appropriate select to the
s3c2412 pm option, resulting in breakage when only compiling s3c2412
support.

Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>