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>

Documentation/devicetree/bindings/display/atmel/hlcdc-dc.txt

 Device-Tree bindings for Atmel's HLCDC (High LCD Controller) DRM driver
 
 The Atmel HLCDC Display Controller is subdevice of the HLCDC MFD device.
-See ../mfd/atmel-hlcdc.txt for more details.
+See ../../mfd/atmel-hlcdc.txt for more details.
 
 Required properties:
  - compatible: value should be "atmel,hlcdc-display-controller"