ARM: digicolor: add low level debug support

Use the USART peripheral as UART for low level debug. Only the UA0 port is
currently supported.

Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Olof Johansson <olof@lixom.net>