ARM: S3C24XX: Move rtc-core.h from plat to mach

plat/rtc-core.h is only referenced from mach-s3c24xx. Hence
move it there to de-populate the plat directory. While at it
also do some cleanup of the header file.

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Acked-by: Heiko Stuebner <heiko@sntech.de>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>