patch 9.1.0823: filetype: Zephyr overlay files not recognized

Problem:  filetype: Zephyr overlay files not recognized
Solution: detect '*.overlay' files as dts filetype,
          include syntax tests for DTS files
          (Xudong Zheng)

Reference:
https://docs.zephyrproject.org/latest/build/dts/howtos.html

closes: #15963

Signed-off-by: Xudong Zheng <7pkvm5aw@slicealias.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/runtime/syntax/testdir/input/dts_zephyr.overlay b/runtime/syntax/testdir/input/dts_zephyr.overlay
new file mode 100644
index 0000000..40d9aa6
--- /dev/null
+++ b/runtime/syntax/testdir/input/dts_zephyr.overlay
@@ -0,0 +1,16 @@
+/ {
+	chosen {
+		zephyr,uart-mcumgr = &cdc_acm_uart0;
+		zephyr,shell-uart = &cdc_acm_uart1;
+	};
+};
+
+&zephyr_udc0 {
+	cdc_acm_uart0: cdc_acm_uart0 {
+		compatible = "zephyr,cdc-acm-uart";
+	};
+
+	cdc_acm_uart1: cdc_acm_uart1 {
+		compatible = "zephyr,cdc-acm-uart";
+	};
+};