dt-bindings: mediatek: add mutex description for mt8183 display

This patch add mutex description for mt8183 display
Signed-off-by: Yongqiang Niu <yongqiang.niu@mediatek.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: CK Hu <ck.hu@mediatek.com>

Documentation/devicetree/bindings/display/mediatek/mediatek,disp.txt

@@ -52,6 +52,7 @@ Required properties (all function blocks):
   For most function blocks this is just a single clock input. Only the DSI and
   DPI controller nodes have multiple clock inputs. These are documented in
   mediatek,dsi.txt and mediatek,dpi.txt, respectively.
+  An exception is that the mt8183 mutex is always free running with no clocks property.
 
 Required properties (DMA function blocks):
 - compatible: Should be one of