powerpc/pasemi: Fix device_type of Nemo SB600 node.

The of_node for the SB600 (io-bridge) has its device_type set to
'io-bridge' Set it to 'isa' so that it can be found by
isa_bridge_find_early() instead of using patches in the kernel.

Signed-off-by: Darren Stevens <darren@stevens-zone.net>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>