tests: gpio_basic_api: add board overlay for em_starterkit

This adds a board overlay for em_starterkit in the gpio_basic_api
test.

Signed-off-by: Daniel Leung <[email protected]>

Author: dcpleung

tests/drivers/gpio/gpio_basic_api/boards/em_starterkit.overlay

@@ -0,0 +1,14 @@
+/*
+ * Copyright (c) 2019 Intel Corporation
+ * Copyright (c) 2019 Nordic Semiconductor ASA
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+/ {
+	resources {
+		compatible = "test,gpio_basic_api";
+		out-gpios = <&gpio0 18 0>; /* Pmod3 pin 9  */
+		in-gpios = <&gpio0 19 0>;  /* Pmod3 pin 10 */
+	};
+};