Add support for using UART3 for DEBUG_LL on exynos.

[dianders: added depend on ARCH_EXYNOS.]
Signed-off-by: Olof Johansson <olof@lixom.net>
Signed-off-by: Doug Anderson <dianders@chromium.org>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>