MIPS: add <dt-bindings/> symlink

Add symlink to include/dt-bindings from arch/mips/boot/dts/include/ to
match the ones in ARM and Meta architectures so that preprocessed device
tree files can include various useful constant definitions.

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

MIPS structures it's dts files a little differently to other
architectures, having a separate dts directory for each SoC/platform,
but most of the definitions in the dt-bindings/ directory are common so
for now lets just have a single "include chroot" for all MIPS platforms.

Signed-off-by: James Hogan <james.hogan@imgtec.com>
Reviewed-by: Steven. J. Hill <steven.hill@imgtec.com>
Cc: Michal Marek <mmarek@suse.cz>
Cc: Shawn Guo <shawn.guo@linaro.org>
Cc: Ian Campbell <ian.campbell@citrix.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Pawel Moll <pawel.moll@arm.com>
Cc: Rob Herring <rob.herring@calxeda.com>
Cc: linux-mips@linux-mips.org
Cc: devicetree@vger.kernel.org
Cc: linux-kbuild@vger.kernel.org
Acked-by: Stephen Warren <swarren@nvidia.com>
Patchwork: http://patchwork.linux-mips.org/patch/5745/


Signed-off-by: Ralf Baechle <ralf@linux-mips.org>