[POWERPC][XEN] Ignore device tree nodes missing "device_type".

[POWERPC][XEN] Ignore device tree nodes missing "device_type".
The latest release of SLOF is missing mandatory "device_type" properties for
the ISA and system-controller busses. Work around it by just printing a warning
and continuing.
CC: Segher Boessenkool <segher@kernel.crashing.org>
Signed-off-by: Amos Waterland <apw@us.ibm.com>
Signed-off-by: Hollis Blanchard <hollisb@us.ibm.com>
--HG--
extra : transplant_source : y%01Ue%D3H%7B%1CL%0DLc%02P%D6%85%AC%E8%1F%07