samples: drivers: dac: add support for gd32a50x

add support for gd32a50x

Signed-off-by: YuLong Yao <[email protected]>

Author: feilongfl

samples/drivers/dac/boards/gd32a503v_eval.overlay

@@ -0,0 +1,13 @@
+/*
+ * Copyright (c) 2022 YuLong Yao <[email protected]>
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+/ {
+	zephyr,user {
+		dac = <&dac>;
+		dac-channel-id = <0>;
+		dac-resolution = <12>;
+	};
+};