dt-bindings: display: mediatek: add property to control mipi tx drive current

Add a property to control mipi tx drive current:
"drive-strength-microamp"
Signed-off-by: Jitao Shi <jitao.shi@mediatek.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Chun-Kuang Hu <chunkuang.hu@kernel.org>

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

@@ -33,6 +33,10 @@ Required properties:
 - #clock-cells: must be <0>;
 - #phy-cells: must be <0>.
 
+Optional properties:
+- drive-strength-microamp: adjust driving current, should be 3000 ~ 6000. And
+						   the step is 200.
+
 Example:
 
 mipi_tx0: mipi-dphy@10215000 {
@@ -42,6 +46,7 @@ mipi_tx0: mipi-dphy@10215000 {
 	clock-output-names = "mipi_tx0_pll";
 	#clock-cells = <0>;
 	#phy-cells = <0>;
+	drive-strength-microamp = <4600>;
 };
 
 dsi0: dsi@1401b000 {