metag: add <dt-bindings/> symlink

Add symlink to include/dt-bindings from arch/metag/boot/dts/include/ to
match the one in arch/arm/... (see the commit below) so that
preprocessed device tree files can include various useful constant
definitions.

Commit c58299aa ("kbuild: create an
"include chroot" for DT bindings") merged in v3.10-rc1.

Signed-off-by: James Hogan <james.hogan@imgtec.com>
Acked-by: Grant Likely <grant.likely@linaro.org>
Reviewed-by: Stephen Warren <swarren@nvidia.com>
Cc: Michal Marek <mmarek@suse.cz>
Cc: Shawn Guo <shawn.guo@linaro.org>
Cc: Rob Herring <rob.herring@calxeda.com>
Cc: linux-kbuild@vger.kernel.org
Cc: devicetree-discuss@lists.ozlabs.org