[POWERPC] bootwrapper: Add CPM serial driver

This serial port is used on all 8xx, many 82xx, and some 85xx chips.

The driver requires that the port has already been set up by the firmware
and/or platform code.

Signed-off-by: Scott Wood <scottwood@freescale.com>
Acked-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Paul Mackerras <paulus@samba.org>