powerpc/powermac: Use sane default baudrate for SCC debugging

When using the "sccdbg" option to route early kernel messages and
xmon to the SCC serial port on PowerMacs, when this wasn't the
configured output port of Open Firmware, we initialize the baudrate
to 57600bps. This isn't a very good default on some powermacs where
both the FW and pmac_zilog will default to 38400. This fixes it to
use the same logic as pmac_zilog to pick a default speed.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>