[POWERPC] Get default baud rate in udbg_scc

On powermac, when open firmware is set to use the SCC for console, this
causes the low level udbg early console to read back the speed and
re-use it instead of hard coding 57600 bps. This doesn't (yet) pass the
detected speed all the way to pmac_zilog though.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>