[POWERPC] Call add_preferred_console when MPSC is console

When a Marvell MPSC (serial controller) port is the specified
/chosen/stdout-path device, call 'add_preferred_console()' so the user
doesn't have to specify a 'console=ttyMMx' cmdline argument.

Signed-off-by: Mark A. Greer <mgreer@mvista.com>
Acked-by: Dale Farnsworth <dale@farnsworth.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>