ARM: sti: Add DEBUG_LL console support

This patch adds low level debug uart support to sti based SOCs.

Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@st.com>
CC: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Olof Johansson <olof@lixom.net>