rtc: rtc-s3c: Add extra option to include RTC for Samsung SoCs

This patch adds HAVE_S3C_RTC to control inclusion of RTC driver for Samsung
SoCs. This option will help to include the driver only for the necessary
machines and not for any given arch.

Signed-off-by: Atul Dahiya <atul.dahiya@samsung.com>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>
Cc: Ben Dooks <ben-linux@fluff.org>