[ARM] S3C: Nove <plat/pm.h> from plat-s3c24xx to plat-s3fc

Move the <plat/pm.h> header to plat-s3c as preparation
for moving parts of the s3c24xx pm support which are
common into the plat-s3c support.

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