ARM: SAMSUNG: Remove platform dependency from samsung.S

regs-serial.h only includes linux/serial_s3c.h. Include this
header directly in samsung.S to remove unnecessary platform
dependency.

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Signed-off-by: Tushar Behera <tushar.behera@linaro.org>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>