[POWERPC] Add bootwrapper support for Marvell MPSC

The bootwrapper requires a serial driver to allow cmdline editing
and information reporting on the console.  This driver is required
by platforms that boot a zImage and use the MPSC for the console.

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