s3c-fb: add support for runtime pm

This patch adds support for runtime pm using the functions.
 - pm_runtime_get_sync()
 - pm_runtime_put_sync()

pm_runtime_get_sync() and pm_runtime_put_sync() are called when
open or release function of framebufer driver is called to inform
the system if hardware is idle or not.

Signed-off-by: Jingoo Han <jg1.han@samsung.com>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>