[POWERPC] Fix mv64x60 early console code to use cell-index property

The MPSC driver and prpmc2800.dts have been modified to use property
'cell-index' as the serial port number, but the early serial console
driver for the mv64x60 has not been modified to use this new property.
This fixes it.

Signed-off-by: Remi Machet (rmachet@slac.stanford.edu)
Acked-by: Dale Farnsworth <dale@farnsworth.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>

arch/powerpc/sysdev/mv64x60_udbg.c

@@ -94,7 +94,7 @@ static void mv64x60_udbg_init(void)
 	if (!np)
 		return;
 
-	block_index = of_get_property(np, "block-index", NULL);
+	block_index = of_get_property(np, "cell-index", NULL);
 	if (!block_index)
 		goto error;