[ARM] S3C: Fix sparse warning from missing 's3c_device_hwmon'

Fix the following sparse warning due to s3c_device_hwmon being
missing from <plat/devs.h>

devs.c:380:24: warning: symbol 's3c_device_hwmon' was not declared. Should it be static?

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