samples: boards: bbc: add microbit_v2 board to pong example

Adding board bbc_microbit_v2 overlay to pong example. This change
mainly enables to buzzer functionality of the example.

Signed-off-by: João Castilho <jpccastilho1999@gmail.com>

samples/boards/bbc/microbit/pong/boards/bbc_microbit_v2.overlay

@@ -0,0 +1,6 @@
+/ {
+	zephyr,user {
+		/* period cell corresponds to initial period */
+		pwms = <&pwm1 0 PWM_USEC(1500) PWM_POLARITY_NORMAL>;
+	};
+};