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

This patch add mutex description for mt8183 display

Signed-off-by: Yongqiang Niu <[e-mail]>
Acked-by: Rob Herring <[e-mail]>
Signed-off-by: CK Hu <[e-mail]>

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

···
   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