openrisc: Add support for liteuart emergency printing

This patch adds support for sending emergency print output, such as
unhandled exception details, to a liteuart serial device.  This is the
default device available on litex platforms.

If a developer want to use this they should update UART_BASE_ADD
to the address of liteuart.

Signed-off-by: Stafford Horne <shorne@gmail.com>