rtc: s3c: make header file local

Nothing outside of the rtc driver includes plat/regs-rtc.h,
so we can simply move the file into the same directory,
which allows us to build the file as platform-independent
code.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Cc: rtc-linux@googlegroups.com
Cc: Alessandro Zummo <a.zummo@towertech.it>