mtd: s3c2410: Use <linux/io.h> instead of <asm/io.h>

Fixes the following checkpatch warning:
WARNING: Use #include <linux/io.h> instead of <asm/io.h>

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>