[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>