[POWERPC] Add early debug console for CPM serial ports.

This code assumes that the ports have been previously set up, with
buffers in DPRAM.

Signed-off-by: Scott Wood <scottwood@freescale.com>
Acked-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>