ARM: SAMSUNG: Update the name of regarding Samsung GPIO

According to gpio-samsung.c, this patch updates the name of
regarding Samsung GPIO. Basically the samsung_xxx prefix is
used in gpio-samsung.c instead of s3c_xxx, because unified
name can reduce its complexity.

Note: some s3c_xxx stil remains because it is used widely.
It will be updated next time.

Cc: Ben Dooks <ben-linux@fluff.org>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>