serial: s5p: Remove common.h inclusion

It's not really needed here anymore. Remove it, as common.h is going
away at some point.

Signed-off-by: Sam Protsenko <semen.protsenko@linaro.org>
Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Minkyu Kang <mk7.kang@samsung.com>