tests: lib: devicetree: Fix devicetree overlay formatting

Fixes formatting of the devicetree overlay to make the new
DevicetreeLinting compliance check happy.

Signed-off-by: Maureen Helm <maureen.helm@analog.com>

Author: MaureenHelm

tests/lib/devicetree/api_ext/app.overlay

@@ -21,6 +21,7 @@
 			reg = <0x20000000 0x1000>;
 			zephyr,memory-region = "SRAM_REGION";
 		};
+
 		test_sram2: sram@20001000 {
 			compatible = "zephyr,memory-region", "mmio-sram";
 			reg = <0x20001000 0x1000>;