arm: dts: overlays: Add external trigger mode support

Adds DT property `trigger-mode` to enable FSIN-triggered frame capture.
Includes overlay and README update for ov9281_trig.

Signed-off-by: Omer Faruk Edemen <ofedemen@lectrontech.com>

arch/arm/boot/dts/overlays/README

@@ -3737,6 +3737,10 @@ Params: rotation                Mounting rotation of the camera sensor (0 or
         clk-continuous          Switch to continuous mode on the CSI clock lane,
                                 which increases the maximum frame rate slightly.
                                 Appears not to work on Pi3.
+        trigger-mode            Enable external trigger mode (0 = normal,
+                                1 = triggered). In this mode, the sensor outputs
+                                a frame only when triggered by a rising edge
+                                on the FSIN input pin.
 
 
 Name:   papirus

arch/arm/boot/dts/overlays/ov9281-overlay.dts

@@ -96,6 +96,7 @@
 		       <&reg_frag>, "target:0=",<&cam0_reg>;
 		arducam = <0>, "+6";
 		clk-continuous = <0>, "-7-8";
+		trigger-mode = <&cam_node>, "trigger-mode:0";
 	};
 };
 

arch/arm/boot/dts/overlays/ov9281.dtsi

@@ -14,6 +14,7 @@ cam_node: ov9281@60 {
 
 	rotation = <0>;
 	orientation = <2>;
+	trigger-mode = <0>;
 
 	port {
 		cam_endpoint: endpoint {