When using a builtin dtb the builtin dtb is built twice, once
from as a dependency of the 'dtbs' target and once as a dependency
of the corresponding dtb.o target. This can happen in parallel
with parallel make which results in build corruption when two
processes try to generate the dtb at the same time. Typical errors
include:

fixdep: error opening depfile: arch/arm/dts/.imx51-babbage.dtb.d: No such file or directory
make[1]: *** [arch/arm/dts/imx51-babbage.dtb] Error 2

fixdep: arch/arm/dts/.imx51-babbage.dtb.d is empty
mv: cannot stat `arch/arm/dts/.imx51-babbage.dtb.tmp': No such file or directory
make[1]: *** [arch/arm/dts/imx51-babbage.dtb] Error 1

To fix this build the devicetree blobs using extra-y instead of
a separate target.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>