serial: s5p: Allow independent selection

Currently support for the Samsung serial port driver is part
of CONFIG_S5P which controls selection of several drivers for
the S5P family.  Give it its own config option such that we
can use it on other SoCs as well.

Signed-off-by: Mark Kettenis <kettenis@openbsd.org>
Reviewed-by: Patrick Wildt <patrick@blueri.se>
Signed-off-by: Minkyu Kang <mk7.kang@samsung.com>