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>