dt-bindings: display: atmel: Fix path to atmel-hlcdc.txt

The relative path pointed to
Documentation/devicetree/bindings/display/mfd/atmel-hlcdc.txt
instead of
Documentation/devicetree/bindings/mfd/atmel-hlcdc.txt

Signed-off-by: Richard Genoud <richard.genoud@gmail.com>
Signed-off-by: Rob Herring <robh@kernel.org>