[POWERPC] Fix legacy serial search for opb bus ports

The patch to legacy_serial.c (1a7507c7

,
Reduce code duplication in legacy_serial, add UART parent types) changed
the semantics for opb ports from type = "opb" || compatible = "ibm,opb"
to type = "opb" && compatible = "ibm,opb".

The result is serial ports on our QS21s (Cell blades) don't get found,
and for some reason the machine doesn't boot at all - possibly it's
panicking due to lack of a console?

The fix is to add two entries to the of_device_id table, one that looks
for type = "opb" and the other compatible = "ibm,opb".

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Acked-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Acked-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>