[POWERPC] Fix console output getting dropped on platforms without udbg_putc

Previously, registering this early console would just result
in dropping early buffered printk output until a udbg_putc
was registered.

However, commit 69331af7
clears the CON_PRINTBUFFER flag on the main console when a
CON_BOOT (early) console has been registered, resulting in
the buffered messages never being displayed to the user.

This fixes the problem by making sure we don't register udbg_console
on platforms that don't implement udbg_putc.

Signed-off-by: Milton Miller <miltonm@bga.com>
Acked-by: Mark A. Greer <mgreer@mvista.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>