[ARM] S3C: Move watchdog system reset to own file.

Move the watchdog reset code from <mach/system-reset.h> to
a new file <plat/watchdog-reset.h> as this code is needed
by both s3c2410, s3c64xx and soon-to-be added s3c24a0.

Signed-off-by: Ben Dooks <ben-linux@fluff.org>