samples: dac: add arduino_giga_r1 overlay

This PR adds arduino giga r1 overlay in the dac sample.
Arduino Giga R1 has a 3.5mm audio jack connected to the
dac.

Signed-off-by: Mario Paja <[email protected]>

Author: mariopaja

samples/drivers/dac/boards/arduino_giga_r1_stm32h747xx_m7.overlay

@@ -0,0 +1,7 @@
+/ {
+	zephyr,user {
+		dac = <&dac1>;
+		dac-channel-id = <1>;
+		dac-resolution = <12>;
+	};
+};