dt-bindings: mediatek: add ovl_2l description for mt8183 display

Update device tree binding documention for the display subsystem for
Mediatek MT8183 SOCs
Signed-off-by: Yongqiang Niu <yongqiang.niu@mediatek.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: CK Hu <ck.hu@mediatek.com>

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

@@ -28,6 +28,7 @@ Documentation/devicetree/bindings/display/mediatek/mediatek,dpi.txt.
 Required properties (all function blocks):
 - compatible: "mediatek,<chip>-disp-<function>", one of
 	"mediatek,<chip>-disp-ovl"   		- overlay (4 layers, blending, csc)
+	"mediatek,<chip>-disp-ovl-2l"           - overlay (2 layers, blending, csc)
 	"mediatek,<chip>-disp-rdma"  		- read DMA / line buffer
 	"mediatek,<chip>-disp-wdma"  		- write DMA
 	"mediatek,<chip>-disp-color" 		- color processor