ARM: S3C24XX: trim down debug uart handling

Using the lowlevel debug uart is a corner case - even more so in a
multiplatform environment. So it seems reasonable to simply let the
developer set the appropriate uart type for the debugged SoC.

Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>