Fix devicetree generation with oe-core master.

After this commit, meta-xilinx will not build for the zedboard.
http://git.openembedded.org/openembedded-core/commit/?id=72980d5bb465f0640ed451d1ebb9c5d2a210ad0c

Signed-off-by: Philip Balister <philip@balister.org>