[POWERPC] Add Marvell mv64x60 udbg putc/getc functions

Commit 69331af7, "Fixes and cleanups for earlyprintk aka boot console",
resulted in printk output prior to the initialization of the mpsc
console driver not being printed.  That commit causes the mpsc's
CON_PRINTBUFFER flag to be cleared since udbg should have printed
the previous output.

I guess we can no longer ignore udbg. :)

This patch provides udbg_putc() and udbg_getc() functions for the
Marvell mv64x60 chips. These functions are enabled if an mv64x60
port is to be used as the console as determined from the device tree.

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