[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>