ARM: SAMSUNG: Replace inclusion of plat/regs-serial.h header file

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

Signed-off-by: Tushar Behera <tushar.behera@linaro.org>
[broonie@linaro.org: Acked for S3C64XX related changes]
Acked-by: Mark Brown <broonie@linaro.org>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>